A very popular thing to do is visiting the Friendship Monument. It is also a good place to do some tandem paragliding. But I recommend to go to the top of gondola to start paragliding there. You can cover a much bigger area for paragliding and the view is amazing. Just as the paragliders for a thermic flight from the top instead of a short flight next to the monumet. There is Plenty of paraglide businesses in Gudauri and they are easy to find online or just standing next to the road usually like 1km uphill from the gas station or at the friendship monument.
Horse Riding happens at the Friendship Monument if you simply want to ride for 10 minutes. If you are looking for a real horse riding trip you have to book ahead with a travel agency.
Quad bike riding is a popular thing as it seems but I never tried and never will. If you just like the experience to go riding one for an hour or so there is plenty of rental possibilities along the road. If you like a real tour better to book with one of the trusted agencies. Dato from Georiders used to offer quad bike trips but stopped doing it because it is to dangerous. But he can definitely recommend someone.