If you live within a reasonable drive of a college or university, check for summer camp-like experiences - you may have to dig a little on departmental websites. Our smallish college runs a summer week for middle-school girls interested in STEM, a robotics camp, a forensics camp, and one for creative writing (off the top of my head). These are all full-day experiences, not overnights. I know our public state universities offer such opportunities, too, including partial and full scholarships for families with demonstrated need.
Depending on your DS/DD's preferences, you might check into him/her being a counselor-in-training for your Boy/Girl Scouts or YMCA camps nearby. High schools and rec centers offer sports camps, but I know those can vary widely. Final thing I might suggest for volunteering is to narrow possibilities down to two choices and try them both out. Be a dog walker at the Humane Society, or volunteer to read to seniors or stock shelves at your local food pantry - something that's meaningful to Kiddo.