Lake Bohinj is an ideal day trip from Lake Bled. The easiest way to get there is by bus, which takes about an hour through pretty villages and flat valleys. A return bus ticket is about 1500 SIT. The bus stops in the towns by the lake, but it also travels alongside the lake to the final stop near the campsite at the far end of the lake and this is a good place to get out for the Savica waterfall and the Vogel.
Vogel is a ski lift cable car -- it's very steep and costs 1600 SIT return. Simply get off the bus and walk up the steep road on the left. Once at the top, the views across the mountains and towards Triglav are amazing. There is a café, as well as chair lifts and walking routes. Some of the chair lifts are active during the summer too to allow you to travel into the mountains and walk back. The ground is uneven, rough and steep, but it's lovely to walk around taking in the mountain air.
After Vogel, you might want to hike to the Savica waterfalls, which are clearly signposted from Vogel and from the lake. Unfortunately, during the summer, they aren't very impressive. There are plenty of other activities to do at Lake Bohinj, but it is much less developed than Bled and so you need to focus on the villages in the area for the amenities; however, this gives the lake a charm of its own and is really worth a visit or stay.