I agree - just wander around. Ljubljana is lovely and charming - and fairly small. Very low-stress. I have found it a great place to start a trip, actually, though it's also a favorite place, too.
Walk along the river. Walk south past the pedestrian area and when you see a creek branch off to the right from the river, follow it a short ways to a big church on the left. Right behind that is Jože Plečnik's house. Plečnik was a key architect who designed a lot of the prominent bridges and buildings you'll see in Ljubljana. You can tour his house with a guided tour - takes about 30 minutes, though it may be too late in the day by the time you arrive to do it that day.
At the church, turn right and walk back into town. This is a less charming walk than the one along the river but gets more interesting as you walk further north.
You can also walk up to the castle if you like. There's a funicular too. You don't have to go into the castle (which, as I recall, is OK but nothing that amazing), but the views down are amazing.
I'd also bring an umbrella with you to Slovenia. I've had a little rain on every visit I've made there (sometimes a lot of rain) even in September. And I've had beautiful, sunny days too. Just be prepared! Even a stroll in a slight rain can be fun in a charming town like Ljubljana.