Sljeme is an ideal place for picnicking, but you may wish to save your appetite for dinner at one of the excellent restaurants on the road home. On the southwest flank of the summit of Mt. Medvednica is a reconstructed fortress called Medvedgrad. The original was built in the 13th century by Bishop Filip of Zagreb, and after a succession of distinguished owners over the next two centuries it was destroyed in an earthquake in 1590. You can wander around the outside (for free), and take in great views of Zagreb. It's a one-hour trek to the fortress from the cable car, or you can reach it more directly by taking Bus 102 from Britanski trg in central Zagreb (just off Ilica, a 20-minute walk west of Trg bana Josipa Jelačića) to the "Blue Church" in Šestine, and then hiking some 40 minutes uphill from there. Take trail No. 12, which is off the paved road past the church cemetery.