Wow! There are some really great ideas here! I think i am going to use some of them. This is one of those topics where I find it much easier to GIVE advice than implement that advice myself, but my mom did some really fun things with us when we were kids. I figure the more ideas you get, the more you have to choose from that will work for your kids.
Dime Jar - (Could be a quarter jar, or whatever ~ INFLATION!)Make a list of jobs for the kids to do. Get 2 jars, 3 when your son comes back. Fill each jar with a certain number of dimes, this can vary by age or number of jobs required. If a job is left undone, a dime is removed. At the end of the week, whatever is left in the dime jar is theirs to keep.
Points - make a poster for tally marks, put each child's name down the side. (You could even put your own name and earn your own points.) Then make a list of jobs and the number of points they are worth to you. When your children do what you ask them to do, they earn points. Then make up a list of priveleges and their point value (ie. 1/2 hr of tv = 20 pts, play with friends = 20 pts, out to dinner with Mom = 100pts, Water park = 500pts, Lagoon = 1000 pts, etc). If they do what you ask by responding, "Sure, I'd love to!" they get extra points. If you do the point thing, too, then if your child asks you to get her a drink and you say, "Sure, I'd love to!" You get points, too! (Lead by example!) Then, if they ask to do something, you ask if they have enough points, if not, help them figure out how to earn more real quick so they can do what they want...it's kind of like saving up money. They have to learn how to budget their points so they can do the things they want to do most. I guess it's up to you if you deduct points they earn, once they use them for a priveledge, or if you just let them keep earning them.
We had a lot of fun with that one.
Tickets~ one summer my mom got together with a few other families (her sister and a couple of neighbors) and all the kids earned tickets throughout the summer. At the end of the summer they put on a carnaval and we got to spend the tickets to buy popcorn and treats, play games, see a video, etc. It was so fun!
If you want more info on any of these, feel free to ask.
Good luck!