From a historical standpoint, Castle Island is significant as the home of Fort Independence, an imposing granite fort built in the mid-1800's. We have yet to actually be there when tours are being held, but even walking around the Fort is interesting to little kids.
Besides Fort Independence, there are several things that my kids & I loved about our two trips to Castle Island.
First, there is the location. I have no hangups about driving in Boston, but the traffic and the expense of parking can make me less-than-anxious about returning. Fortunately, Castle Island is an easy drive up Route 93 (we live south of the city) to exit 15; signs point the way from there. The main road, William J Day Boulevard, runs alongside the beach and literally ends at the Castle Island parking lot. And, get this, the parking is free. Free!
One of the first things you may notice after parking is the low building at the far end of the lot. This is Sullivan's, a seasonal take-out restaurant that sells hamburgers, hot dogs, fries and other kid (and adult) favorites. It's cheap, greasy and crowded. . .in other words, perfect. There are outdoor tables or plenty of grass nearby for a picnic.
After eating (doesn't everyone eat first?), follow the path past the bath house to the playground.
The playground has two large play structures and swings for kids of varying ages. Park benches line the perimeter, so if you are lucky, you may actually be able to sit back and watch your kids play for a while.
Wait, what's that noise? Oh yeah, I didn't mention the aspect of Castle Island that made my son wild the first time we went. The planes. Planes from Logan zoom overhead every minute or so, and they are low and loud. My little guy channels this guy the entire time we are there, and I never get sick of it.
Besides the playground and the planes, there is the beach. When we visited in July, lifeguards in their chairs lined the beach all along the road leading to Castle Island. There were kids in the water everywhere we looked. On our last visit, it was hot, but the water had cooled significantly, so there were mostly just toes getting wet (one other thing is that it is windy here; pack accordingly. And your hair? Fuhgeddaboudit).
Walk along the beach a bit to the rocky part. Over here, my kids found the bodies of crabs, clams, mussels, and loads of shell fragments and cool rocks (bring a pail or deep pockets).
Although the warmth of summer is gone, Castle Island would still be a pretty great place to visit on a fall day. It bustles with dog walkers, moms with kids, runners and retirees---all out enjoying the neighborhood beach with the planes overhead.