I quite like microstates and small countries in Europe.
Has anyone travelled to Liechtenstein? Liechtenstein is surrounded by lesser-visisted parts of Austria and Switzerland that seem amazing and I love the valleys and the mountains in Liechtenstein itself as well. It is also close to very tourisy and attractive Lake Constance (Bodensee). I bet it's prohibitevly expensive but I wonder if it's worth it a day trip from Germany, Austria for example?

Yes, I've been, a fair few times
No it is not interesting, unless you like to see whole streets lined with the names of accountants
Yes it is expensive, no really, things are very expensive

Lake Constance is interesting enough, I've cycled around it, it feels very wealthy, very old and very dull.
The Swiss side is worse.

But if your idea of a great time is eating cake and chilling it might be ok

I've been once, one day. It really is quite dull. The views are nice, but not outstanding for the region. If you're already in the area (Sankt Gallen in Switzerland and immediate surroundings, or maybe Voralberg in Austria), it might be worth going there for the novelty. And the area is indeed worth visiting (especially the Walensee, Saentis, Chur). I wouldn't go out of my way to visit Liechtenstein from Germany (unless you're already on the eastern Bodensee shore for other reasons), or from Austria beyond Voralberg.

It's expensive, of course, but in line with the wealthier parts of Switzerland (like Zug), I think; I haven't slept or eaten at a restaurant there, so I wouldn't really know.

I've been and have driven through/by once or twice since when on buses headed elsewhere. Like you and most visitors, I went because I was curious and it seemed novel. Can't say it was all that exciting.

I bought a sandwich, it hurt to pay for it

It's pretty but not that different from the part of Switzerland it adjoins.

Worth it if you're in the area, not for a massive detour.

It's easy to visit Liechtenstein by public transport (consisting of Austrian trains and Swiss Postbuses).
The local currency is the Swiss Franc.
The local dialect doesn't really differ form that spoken in neighbouring Switzerland (Buchs SG) or Austria (Feldkirch).
Same remark for the local cuisine and regional dishes.
There are some strenuous hiking trails.
Liechteinstein has even a Alpine Club Hut, (Pfälzerhütte) on the trail.to Scesaplana Hut (Switzerland) - Totalp Hut (Austria) - Carschina Hut (Switzerland) -Tilisuna Hut (Austria).