Start with a roof covered sandbox with a screened cover to keep cats out, and a sun/rain shade/roof. Later add four posts if they're not already there for the roof, to support a platform that can serve as a fort. Add a roof up there as well.
A simple ladder and maybe a rope or pole for alternate entry/exit. Try not to make it too specific, meaning don't try to make it a boat or a castle, let your kid do that with her imagination.
Consider getting a large diameter plastic drainpipe, say 18" diameter and 4-5 feet long. At first it can just lay on its side, but later can become a tube slide or climbing tube.
If you forego all the safety railings etc. like I did, be prepared for your kid's friends' parents to be a little worried about it. They'll deal. We simply had a rule for my daughter's treefort, you had to be able to climb up the ladder on your own in order to allowed up there. And a limit of 2-3 kids until they were a little older and had good balance etc.