This seems like a really silly thing to fall out about. It's just one aspect of what sounds like a lovely trip!
Is he clear you are happy to organise the gondola ride and pay for it?
If so, maybe he feels like he will feel silly and self conscious, just the two of you on the gondola? (I'd feel a bit of a twat I think, and a lot of the guys I have dated certainly would have .....). You need to ask him what it is about it that puts him off. He is allowed opinions - it's his trip too!
You seem to have taken huge offence to him suggesting you lower your expectations, but I actually think it's good advice. I have often found that if you go into an experience expecting it to be perfect in some way, it can easily fall short and lead to disappointment. That's not to say you should never look forward to anything, but be aware that life can throw up the unexpected.
For example, I'm just back from a trip with my daughter, and one of the things I enjoyed most was going to a museum - I'm not a huge fan of museums generally, we only went because it was raining and we couldn't do the activity we wanted to, but as it happened the museum had some stuff we were both really into......
Going somewhere with an open mind can often lead to you having the most fun, I think!
But anyway, don't fall out about the gondola - just make it clear you are very happy to pay for and organise it, and if he still doesn't want to go, you need to listen to why.....