Congrats!
I know your head is probably spinning with all the advice (but see, thats how passionate people get about NYC -you and your soon-to-be-fiance, will be the same, come Feb!) but I just wanted to add my $0.02!
Met a couple over there in March, his plan was to do it in the Empire State, but he got up and it was PACKED, said that it just took away from it. He then planned to do the carraige ride in the Park...and he ended up fit-to-burst, doing it in the hotel room! He didn't say as much, but I got the impression that he hadn't
really enjoyed the first couple of days, with nerves...
My main advice is to take it as it comes -enjoy it! If you rush around too much, you'll be too wrecked to enjoy it. Have a list of 'possibles' and just do them if it suits/whatever fits best with the weather etc.. - my favourite part of NYC is just wandering about, dipping in for a coffee or a beer or a bite to eat and then wandering about a bit again - its such an easy city to navigate and you don't have to 'do' everything, to get a sense of it.
Best of luck - oh and I'd avoid telling your FIL to-be that you're hitting the Credit Union for a couple of
boob
PB
P.S - Why not by a costume-jewellery ring in Topshop/Awear/Dorothy Perkins...somewhere cheap, to do the question popping bit!