Velebit mountain is the largest mountain of the Croatian Dinaric Alps. On the coast side it offers amazing rock climbing as it is basically a sheet of rock rising out of the sea. The east side offers an incredible contrast, rolling lush fields, gentle green hils, Colorado-like landscapes dusted with sheep and horses.
This area is still fairly undiscovered by the tourist hordes. Besides Plitvice lakes (amazing!), tourist industry there is in its infancy stage. It takes a little forethought to arrange a place to stay and activites, but some very creative projects have started to pop up and any of them offer a wide range of possibilities. There is a paintball course close to Gospic, lost of accomodations and tours close to Plitvice lakes, and some tour agencies from the coast offer day trips to Lika.

You should check out Linden Tree Retreat Center, a gorgeous horse farm with Native American tipis as accomodations. They offer horse back rides into Velebit, trips to the caves (this is karst country, so there are gorgeous caves all over), horseback trips to the sea, or day trips to the beach by car. They are really affordable (about $50 euro per day) and the services are all inclusive.
Be sure to see the mountain from horseback, and to dive into the Lika river. This is some of the most pristine drinking water in Europe, and it also makes for some amazing beer. Check out the local brewery in Pazariste (Velebitsko Pivo).