I went to New York with JustYou and it was amazing - I ended up going to Venice with a lady I met on that trip too as we both wanted to go and didn't fancy it alone so that worked well. I've gone spain for a day (oh yes it was brilliant) to dip my toe and like you I'd be fine in the day but what to do at night? I need to brave it and try Majorca or somewhere I know for a few nights first I think
To add - if you ever decide on Venice, it's a 2-3 night tops place. I did 5 and I was SO BORED I couldn't wait to come home
it's beautiful but very small