Reflecting on all the afterschool activities my kids have taken part in this year on the island, I'm so grateful school has stepped in to provide so many enrichment opportunities for our curious kids. For some kids who are older or passionate about certain things, even the broad offerings at school don't quite fit or are at an age-inclusive or beginning level. In these cases we have done some research and found a plethora of activities in which the kids have participated over the years. Most of the summer programs have financial aid/campership options. I'm going to list some that we've found, including some summer intensives, and hope people will use the comments area to add community programs they've found helpful.
Technically your children are entitled to participate in your home school district's extracurriculars, specials, and classes if you can fit them into your day. This includes sports and clubs of all kinds. Most sports go through sixth grade as community services activities, then move over to middle school teams in 7th grade. This is different in each town, and is changing as school budgets get slashed, but contact your community services department to find out how it works in your town. The Maine Principals' Association rules for sports are that you may participate in the sports program of another community if your home district or school does not offer a similar program. This really refers to middle school and up and is designed to keep kids from hopping around between the best teams or, as we like to say, where the snow is whiter. We have participated in Nordic skiing this way in Portland and lacrosse in South Portland, and the boys have done both afterschool golf and VEX robotics (neither of which goes by MPA rules, as they are community services sports) in Cape Elizabeth.
Coastal Maine Aquatics swim team
Musica de Filia girlchoir (also, Boy Singers of Maine for those so-inclined)
317 Main Street Community Music Center in Yarmouth for music lessons, also the Portland Conservatory of Music
Skating lessons at Portland Ice Arena and Family Ice in Falmouth
summer sailing lessons at SailMaine
Maine Robotics Camp at USM
Also, as you all know, we found out that our high school robotics team takes middle schoolers with their principal's permission and grade checks, and it's been a time-intensive yet fun winter.
Summer X at Maine School of Science and Mathematics has been a godsend for our techie, and has programs for boys and girls in grades 5 - 9 (separate weeks for boys and girls, also the 7-9 boys have a separate week from the 5- 7 boys).
The Maine Conservation School at Bryant Pond teaches an array of such fundamentals as bowhunter safety, outdoor survival, fishing, and primitive skills; Tris calls it heaven.
LLBean offers a 'free' fly-tying workshops for all ages (fine motor skills help) every Friday evening at 7:00 from January to April. I put the 'free' in quotes because you do end up spending about $15 a week on the supplies you need to tie the particular fly being taught. However, you will end up with lots of materials and knowledge to move forward, and if you go with friends you can share supplies.
Friends Camp in South China, for budding peaceniks! Graham had a great week (pouring rain) a few years ago but never went back because he became intensely involved in other things, but a small contingent from FSP heads up there every summer.
The Maine Rock Gym has a winter climbing team for kids aged 10 - 18 and afterschool programs for ages 6 - 15. Graham did the climbing team a couple of winters ago. The meets are a hoot, like a frat party (no beer) with people dangling over your head and a special sweaty-foot ambience.
As younger campers the kids all did day sessions at Camp Ketcha in Scarborough. They've added more specialty sessions for older kids recently, so you should check them out. They also have afterschool ropes, archery, and horsemanship sessions in the spring, as well as school vacation camps.
I hope this helps as you plan for your extra-curious kids. Please do let us know of any enrichment gems you've found.