We thoroughly enjoyed our visit to Slovenia 2 years ago and can't wait to return. I agree about getting a hire car, it certainly opens up your options of visiting places off the bus routes etc. We stayed in Lake Bled which was ideal for us, nice walks around the lake, good cafes/restaurants, interesting castle and church on the lake to visit. We stayed in Hotel Jelovica which was ok, have a look on Trip Advisor for the latest recommends.
Here's just a few places we enjoyed visiting:-
Lake Bohinj, not a long drive from Bled and this can be linked with a trip up the Žičnice Vogel cable car for a wander around the mountains with amazing views of the lake and beyond.....not sure about in March what the mountains will be like, but worth enquiring.
Postojna Caves and Predjama Castle. The caves are a very touristy.....but they are huge and well worth a trip, nothing like we have over here by a long way. The cave is very photogenic, both of these can be a linked visit.
Ljubljana Old and new town worth a day visit and easy bus ride from Lake Bled, we did this rather than take the hire car.
Vintgar gorge, lovely 1.6Km gorge with a timber boardwalk and bridges for access. Located near Lake Bled.
Triglav National Park is amazing, no need for high level walks to appreciate the small villages and scenery.
I hope the above helps a little…...enjoy.