We usually don't go into the parks the first night unless we get there early in the afternoon. There's so much to do and explore at the Fort!
As some have mentioned already, the campfire sing along is a good go-to, but bring the bug dope if they bother you (they love me!). The someores can be a sticky business and as far as I could see there aren't nearby restrooms to wash up, so you might want to bring hand wipes.
If you're not making your own dinners, be aware that Trails End, the restaurant at the Settlement is, in our opinion, one of the best restaurants on property. As such, reservations fill up pretty quick. Sometimes they can fit you in, but it can be a wait. Getting and ADR is worth it.
There are two swimming pools--the Meadows is the main pool. I don't know if you have kids, but they'll often have things for young'uns to do. [Edited to add] The "Swimming Hole" (I think it's called) is a smaller pool, and from what I could see, more quiet. I haven't gone there, but my s/o thought it was ok.
The Meadows store is the better of the two stores in the Fort, especially for extra camping/RV needs. The other store is the Trading Post at the Settlement. Both stores have selections of frozen foods, ice cream and goodies, basic needs, etc. If memory serves, I think the Meadows again is more well stocked with groceries then the Trading Post. You're going to pay a premium for these items, but they come in handy if you forget. Just as a heads up (because it was an issue for me), while they carry a pretty good selection of over the counter drugstore items, there are some things like Lactaid, Allegra they don't carry. They also don't carry Lactaid type milk. Your best bet is, as mentioned upthread, to buy as much as you can in advance.
There are carriage and wagon rides for an extra cost. I've never done that but they look like fun.
We love to simply walk around the Settlement at night when the weather is good; it can be a lovely way to relax and unwind after a long drive and setting up camp.
There's the fireworks and water parade at night too--don't recall the times, though.
There's so much to enjoy at the Fort, we've considered just going down there simply to camp until all the construction at the parks is over.