When you say 'My "non conversation" classes are fine', are these the same classes as 'Their other English lessons' ('taken by Chinese English teachers who work strictly from a textbook where rote repetition is the norm')? I am presuming that they are (it would be unusual if you had more "sole" responsibilities than the conversation class), and that in this "one and the same" non-conversation/taken-led-organized by Chinese teachers class, you simply team-teach/assist/try not to look too bored.
Anyway, perhaps you could try looking at that textbook and seeing if there are any useful examples at all in it; if there are not, you could try adapting the useless or silly examples into more genuinely (DIScontextually-speaking) amusing/strange/ridiculous examples, or if that is still hard work, fusing two sentences to produce the bizarre result.
Examples: Are you coming to the party?
> Are you flying/swimming/jumping/hopping to the party?
(Okay that wasn't so funny perhaps).
Are you coming to the party?
+
What'
s your dog's name?
=
Is your monkey coming to the party?
(Again, not hilarious, but I am thinking off the top of my head here. At least you have a book, and a silly one to start with, from which you can draw and alter examples).
Note that the grammar is still perfect in every case, regardless of the nature of the transformation (testing the rules of humour, rather than those of syntax. Humour is probably something Chomsky should consider introducing into his method - he might even end up with some sentences that would make more sense than many of his "stupid", silly AND BORING, ones ever do).
Note also that you could add things like 'your mother and her pet monkey/elephant/hamster etc', or 'Are you AND your mother coming to the
fancy dress party/brothel/orgy/graveyard?!'

(Okay, I admit, some of those may be just a little too bizarre or racy for JHS kids once you've quickly explained the meanings by gestures etc.

).
Recombining the previously introduced elements is also an option, as that 'Are you AND your mother...?!' shows. BTW, it's primarily you, and not the students, who thinks up the funny sentences to begin with, but if they want to have a go at making up their own once they can see how you're thinking, you could let them try maybe.
A freer/easier way than sentences would be a word jumble (think, "cut-up substitution tables", where beforehand/before cutting them up

, every cell was filled with a word - so that in principle equal numbers of equal-length sentences are made later by students arranging the individual words - and where as many cells as possible i.e. in the "content/open-set" as opposed to the "function/closed-set", were filled with silly, "interesting" words ('interesting' does not need to be 'archaic': banana' is interesting right away. 'The teacher threw the banana/brick/grenade at me').* -See end of post.
Such a sentence building activity would need to have some rules e.g. Begin making "silly sentences". Begin each one with a 'The' card. (Play then passes to the player on the left. They can lay a card down, or pass. This is the "card game variation"). Each sentence should have 7 words in it - that is, try to aim to make five sentences. Call the teacher if you are not sure what card to put down next. You will have five minutes. When 'time' is called, you must stop playing and wait for the teacher to look at your sentences. Groups will be awarded a point for every correct sentence they make. An additional point will be awarded for especially funny sentences.
The game is co-operative in nature, working together to beat the clock and win. You would need to decide if sentences such as 'The teacher threw me at the banana' are correct or not. I doubt it: although the aim is certainly to have fun and not worry too much, 'The teacher threw the banana at me' ultimately makes more sense. So, nil points for 'me at the banana', 2 points for 'the banana at me'.
There may be possibilities either way, given a certain 7 cards, that would be worth exploring and both ultimately not only correct but interesting too. For example, 'The teacher/tiger threw the tiger/teacher at me'. It could even be a lot of fun to act out these different senses to explore/establish their meaning: 'The teacher threw the tiger at me' - Is the teacher a tiger-trainer who wants the tiger to maim the student - teacher points and indicates what needs to be done to the student by the tiger, which is perhaps a cowardly tiger; or was the teacher attacked by a tiger that they then "passed" like a hot potato to the student, also part of the safari group?
Students could even sketch what they think is being conveyed by the rest of their sentences - that is, the ones that you indicated were correct - once you've acted out a few of their first ones and they can see how you're approaching the sentences - as holding and conveying possible meaning(s)).
I could go on, but I won't, lest you shout at me for being stupid and unhelpful to boot. And the length of the post is no indication of the quality of the ideas...I'm just showing you how I run with ideas. This is the sort of typical instance in which I have put in an hour or two thinking, and where I then would, in real life, spend a few hours developing materials, then possibly a few lessons trying to improve the activity before finally scrapping it.
I'd just like to add that, if the non-conversation classes are relatively ordered, orderly affairs, you might consider adopting their approaches (and, in connection to that, their "language", but subtly altered to produce
interesting behaviour/resonses/>eventual communication?), at first to the whole class and, if the response is okay, then in terms of worksheets for pair or small groupwork.
(Actually I would be tempted to give them any fun handouts as quickly as possible, in case a long explanation or demo by the teacher and class/nominated students itself turns into a drag for them, despite the potential fun factor. Try to get their attention with that big "shock", then get the handouts distributed and the activity up and running as quickly and efficiently as possible. Some more guidlines along these lines can be found at the link immediately below. The links after that help provide more context).
http://www.eslcafe.com/forums/teacher/v ... 0680#10680
http://www.eslcafe.com/forums/teacher/v ... php?t=2149 ('Guess the movie' game)
http://www.eslcafe.com/forums/teacher/v ... php?t=2122
http://www.eslcafe.com/forums/teacher/v ... .php?t=912 (about serious vs. fun)
http://www.eslcafe.com/forums/teacher/v ... 1947#11947 (my own suggestions for a "serious" curriculum)
You may have read most if not all of those, but hey, what's wrong with re-reading stuff?
I'm imagining that the students won't mind being silly with the English, rather than being 'serious, oh no not AGAIN' they groan, about something seemingly "interesting" to and vital for only the teacher, which in today's class is called "conversation", 'yuck!'. Perhaps if they are allowed to take it a little less seriously/actually have some fun at first, they will more eagerly get into the habits you were previously trying to simply force them into, after which you surreptitiously and subtly start upping the ratio of more "serious" forms-functions-activities.
How does that sound? I haven't done anything like that explicitly, but this way of thinking (avoidance?!) is a part of my approach in less-than-ideal situations (and which aren't in some way or another!), and I have always had "some" (usually noticeable improvement, at least, given time, patience, humour and above all kindness - humour minus any sarcasm) measure of success (some teachers on Dave's seem to have reported things getting worse as a result of them trying to "get tough" or lay down the law).
Threats (not that you've made any yet, but you might have been tempted to) are a pain in the ass for everybody, especially the teacher, ecause they then have to actually inflict the punishment or lose face. Better to lose face gradually...by resorting to humour!!! Forget self-respect! YEY!
*Hmm, all these slashes also have reminded me that sentence stems,
with a silly potential choice heading the gap, might be an "easy" option, and would certainly look a bit more more inviting than the usual "test" items these kinds are probably sick of by now:
The teacher threw the
banana/.............../.............. at me.