I'm headed to Aruba from 8/6 to 8/13 for the very first time! I'm traveling with a friend who isn't as "outdoorsy" as me and I need to find someone to go horseback riding with me. Can't book private tours for just one person apparently. Let me know if you might be interested!
Actually, if you call somewhere like La Ponderosa, they'll be happy to come by and pick you up at your hotel, and yes, you can go by yourself. That is, they'll take single person reservations. If you're looking for a private one person tour, I'm pretty sure they will do that for you. They tend to do all they can to accomodate the tourists, and the horses and guides on the ranch are excellent.