The largest and quite possibly the best-equipped campground in Slovenia is in Lesce, about 2.5km northwest of Radovljica. Situated on a small lake near a bend of the Sava Dolinka River, the camping resort offers no end of summer activities and recreation facilities: beach, playgrounds, bike hire, swimming, fishing, guided walks. The bungalows (really timber chalets) are top quality.
Note that prices listed are for the July to August peak; it's cheaper outside this period. Cyclists and backpackers get a discounted rate.