In addition to rather frequent buses between Ljubljana and Bled, there is often a taxi hanging around the Bled bus station, looking for folks wanting to save a bit of time on the trip back to Ljubljana. I don't remember the exact charge (perhaps 7 euros?), but if the driver is able to fill up the cab, it will be only a very few euros per person more than the bus. There's nothing wrong with the bus, but since your visit is short, you might find it worthwhile to save that 20 or 30 minutes (or whatever it is). Unfortunately, there don't seem to be taxis at the Ljubljana bus station trying to put together a carful of folks wanting to go to Bled. It might still be worthwhile to check on what a cab would charge you.
I shared a cab from the Bled bus station to Lake Bohinj, and it was reasonable for the distance covered; I believe it was 9 euros per person in a cab with 4 passengers. The trick is to know whether you are potentially going to take a taxi, so you don't buy a bus ticket too early and end up discarding it. The taxi probably won't show up at the bus station very long before the bus is due to depart.
In general, I found taxis in Slovenia to be a real bargain. The Ljubljana tourist office told me something surprising: In the city, you will save money by calling for a taxi rather than grabbing one on the street. I think it's something to do with when the meter starts running.
Ljubljana is definitely worth some extra time, so you wouldn't go wrong by staying there and day-tripping to Bled/Bohinj/etc.
The Ljubljana bus station is quite near the train station and about 1 km. from the edge of the old town. There is sometimes a line at the ticket counter; you can't show up 5 minutes before bus departure time and be sure you have time to buy your ticket. I don't think buying on the bus is an option. The fellow selling the tickets last year spoke very good English, as do most of the young folks in Slovenia, so you should have no problem communicating. I believe the tourist office had copies of the bus schedule for Bled to hand out; in any case, it was very helpful. I'd definitely buy the Venice ticket ahead of time. There's no reason not to do it on the day you arrive in Ljubljana. I'm always just a bit nervous about a sell-out when dealing with buses that cross borders.