Coloring, simple art projects, reading to them...
The most important thing for you to give these children is a set schedule. No matter what you do, do it with consistency and make sure you plan the day properly.
If you have a local park that you can take them to for an hour every day, then take them (make sure the parents are okay with you taking them places).
At 2 1/2 both girls should still be getting naps but the boy won't. While the girls are napping you can set aside 30 minutes or so and read to him. The "special" time will be good for him. Like Brad suggested, you can also let him "help" you with housework.
One of Kait's very favorite things to do is help me fold clothes, seriously. She also likes to help unload the dishwasher. At 5, 6, and 7 putting silverware away and folding a stack of washcloths really isn't a big deal and will give him a sense of accomplishment while teaching a little responsibility.
You are right in not allowing too much time for the television. It's not really beneficial to their educational development and too many parents rely on it as a "baby sitter".
One of the decisions I made early on was to focus on getting toys that were of an educational nature. Kait has a VSmile rather than a Playstation. She had a Leapfrog rather than a gameboy. And, believe it or not, I had more teachers and other parents tell me that she was really advanced for her age. If you focus on games that will stimuate their mind rather than just entertain them then you'll be doing more for them in the long run.
Spend some time every day doing something active. It'll be good for you and the children. Let them have TV time while you are doing something they can't help with. But, for the most part, just try and relax and spend some QUALITY time with them.
Good luck. I did the whole babysitting for a friend thing, it isn't easy and it's certainly not always "fun". Kids are a handful, especially when they don't belong to you.
OHHHHH, one last thing, set boundaries and STICK to them. Their parents need to know that you have rules and you need permission to enforce those rules and then enough backbone to do so. Don't let them run your household or you'll end up hating every minute of the 12 hours they are with you.
Again, good luck, I don't envy you.