We will be in Ljubljana in early May and I see that the Jože Plečnik House now has online booking. Is it necessary to book tickets for any of the attractions (museums, castle tower, churches, TI guided walks) online in advance? Or, are same-day or next-day tickets readily available?
I recommend you pre-book castle tickets, as the ticket line was rather long when we went. Otherwise, we pretty much just walked in everywhere. We used Ljubljana Free Tour (something of a misnomer--1 free tour per day, plus several scheduled tours daily, and private tours available). Its website claims that prebooking isn't required for its free tour. We did a private tour, for which prebooking is required.
We did one of the "free" guided walking tours, and we did not book in advance. We just showed up at the appointed spot. There were quite a few guides, and each guide took x number of people. Our guide was very good, and of course we tipped her. That is how they make their money.
It’s not a situation where you’d be in danger of missing out, and need to book far in advance. But as a convenience it’d be useful to skip lines, or just lock in your schedule a day or two beforehand.