In the Samokov Hotel there's a big board to the right of the main door with lots of excursion posters on. There's a tourist information place in the lobby as well. It didn't seem to me that you had to be a hotel guest or on a package holiday.
Also at the Hunters/Balkan-PMS office by the Flora Hotel they can book you on tours.
Every time I go to Borovets I think I will take a tour to the Rila Monastery because it is fabulous, but I've not managed it yet!