Gillette Castle looks like a medieval fortress, but inside the stone castle you'll find a beautiful home built by William Gillette.
I used to go here a lot when I was a kid. The trails are great to hike on and the castle itself is also interesting, but it isn't always open for viewing. I would call ahead and make sure that it is if that is why you were going. My 3 year old wasn't that into the museum at all , I think it is more for older kids. You can take a trail down to the CT river and it is very relaxing.
We enjoyed our time at the castle. The youngest got a little frustrated because you aren't allowed to touch anything (walls included). But the presentations are quick and interesting. The kids enjoyed reading the explanations scattered around the rooms. The grounds are beautiful, take time to follow the trails, the lily pond in bloom is gorgeous!
Small kids probably won't appreciate William Gillette(Sherlock Holmes) home but it's a great little spot. If you are lucky you will be able to get a tour of the house(sometimes it's booked or they may not be offering tours), even if you can't, enjoy all the trails and water views(if you geocache there are generally several cache's there). I grew up in Ct and Gillette's Castle has always been one of my favorite spots. I would go there several times a year(even in the winter), It was actually one of mine and my husbands first dates, and he proposed to me on the balcony overlooking the water.
Our family makes it a point to hike "the castle grounds" at least once a year. Located within a reasonable distance from Devil's Hopyard, another great hiking spot, the Gilette Castle State Park is one you have to really prepare yourself for. For those traveling with kiddos ages Really Wee to Barely Toddling, you'll want to make sure you have a baby carrier ... and I don't mean the type that attaches itself to your MacLaren.
Before visiting, make sure you get in contact with the staff at the castle. We have visited several times in hopes of being able to see the interior of the castle with zero success. Keep in mind we've been hiking this area for over 8 years.
There are plenty of picnic tables in a covered area just below the castle walls (directly behind you if you are facing the castle entrance). Make sure to have a few Kodak moments with the family (this is where easily manipulated tripods are most handy) along the castle wall that overlooks the Connecticut River.
Note: there is an ice cream "shop" open during the summer season. Make sure you call ahead before counting on it to be open. This place is a step above "off the beaten path" so local eateries are not quickly accessed.