Why don't you check out this video: http://www.youtube.com/watch?v=_EbQDrLwkxo
It's with my friend, Julie, who uses a lot of games and group activities with older kids, but I bet you can adapt them to your 5-year old in private lessons as well.
Among the activities you'll see in the video:
1. Warmups - good physical activities for students to gain awareness of their bodies' capabilities
2. Rhythm tapping at the table - builds up listening skills and rhythm. Also an opportunity to work on the bridge in the hand.
3. Tempo cards - put them in order, and then let the kid(s) walk/run in the tempo that you tell them to.
4. Tempo guessing - You can play a piece at a certain tempo and see if he can guess it, or vice versa.
5. Rhythm cards - Make a bunch of rhythm cards and let him put it in any order and then chant or tap them out. In the video, Julie uses 16th note rhythms, but you could just start with quarter notes, half notes, and rests with your 5 year old.
6. Flash card hiding game - You can take those flash cards you have and hide them and let him go find one at a time and bring it to the piano and play it as fast as he can. This is particularly effective for the 'hyper' kids.
7. Improvisation - You can still let your kid improvise like on black keys or other limited key range.
Let us know if you use any of these and how well they work!