I mean, beyond the small hobby farm + live-in help, of course!
I'd buy a new truck and trailer, first off.
Trailer: custom Hawk 2h straight load goose neck. I'd expand the dressing room a bit so there was plenty of room for tack trunk as well as my own stuff. I'd make sure it had a water source underneath the goose neck, and would plan the high portion so that it made a comfortable bed.
Then I'd replace my truck, which would go in honorable retirement to my parents where it could live out life as a weekender truck.
I'd get a brand-new, kitted for towing 3/4 ton GMC Sierra. I like GM trucks for towing. I'd get the extended bed, and not the full extended cab - just the king cab. Seats in the back, but not separate doors. 4WD, without question. Honestly? I want exactly my same truck, brand new, with 4WD. I love my truck.
I'd find a reputable storage place to garage them both over the winter so they don't get wear and tear.
Other than that...?
Both of Tristan's saddles fit him and me, and I love them. Ditto his bridles. His blankets are all in good working order. He gets the food, supplements, and medication he needs. I might consider doing blood work quarterly, instead of bi-annually.
If he pooped money, I'd pay down my mortgage faster and build up his emergency fund. I'd increase my cash flow so that I could take lessons weekly over the spring, summer, and fall.
I'd probably seriously consider investing money into my current barn. Maybe I'd be some kind of partner in the business so that they could do some major capital improvements. I honestly think the property is marvelous as it is, and they do a nice job maintaining it - but I also know that more money in a horse property is never a bad thing! Or maybe I'd invest in a fancy horse for my trainer, and fly down to Florida to see it in the big classes. Who knows?
Finally, I'd sink some money into improving equestrian sport. I'd either establish a regular fund or regular donations for a couple of horse rescues, and I'd set up a decent sized scholarship for young riders. Probably through Pony Club, aimed at New Englanders.