I teach very small groups privately and you just need to add twists to all the usual games.
Make things
timed races where the students compete to beat their old times. Use this for words, short phrases, phonics, reading, etc. but don't kill it. I generally use timed races to increase fluency (the students already know the material. I just want them to get it out faster.) Timing them means they compete against themselves and you have less of a problem with the stronger one. Compete yourself and be dramatic about losing or winning. If they are not competetive, please scratch this idea.
Othello is a great game to play in low numbers. Designate a 6X6 board and use your flash cards (front and back, not black and white.) You can have the kids make sentences, ask questions describe something using the cards. In this game they will repeat the same things over and over again, so make sure the language is challenging enough that it doesn't seem ridiculous, but not too difficult that it distracts from the game.
I try to think of games where the object of the game/activity is not to perfect English but the students use English throughout the activity.
A walk is a good idea, but keep it cool. You said you had some extra lesson time. Ask them what they want to do. Where do they want to go? Then just see what language develops (probably a lot of simple words/phrases like - cool, nice, you gotta be kidding, what now? etc. - Things that are good to know and can kill conversation if you don't know them.)
Reward their effort. Stickers are my system of choice.
I hope that helps or gives you some ideas.
Mark
www.mes-english.com