Hi Catie,
Day camps are a good place to start. Most local community centers have day camp programs geared toward older children and young teens, with more of a focus on field trips and specialized interests like sports or performing arts. Of course, day camps come with a price tag, so teaming up with a parent (or a few parents) of your child's friends can also be a good idea as long as you're all on the same schedule, and you can all trade off watching the kids for a few days a week. Or get your kid involved with volunteer work -- there are some groups that accept help from teens, and may even have special summer programs for them.

Hi Catie!  I also have a 10 yr old.  i am going to be putting him in day camp.  We went last summer to check it out and he is really looking forward to going.  They have all sorts of activites for them and it looks lik a jam packed day of fun and games.  it will keep them busy and they will have tons of fun!  Good luck!!!