We had our honeymoon in Orlando in 1999 and also opted for no car. It was a bad mistake. If you are staying on I Drive you might get away with it but we were on 192 in Kissimee with roadworks galore (and no pavements) and it was very difficult to get about. We even had to get a taxi to TGI's which was 10mins walk up the road but the other side.
If you are considering a car, I would go for it. Even on I Drive. The freedom it gives you to get to other areas for restaurants, attractions etc is so worthwhile.
As Salisburygal has mentioned, do you mean the I Ride Trolley on I Drive for tickets? There is some confusion on other forums at the moment about how much of an attraction International Drive is. Some people are thinking it is a place to visit (similar to a theme park) and are allowing a day "to see it" but as I have pointed out countless times I Drive really is primarily a strip of hotels and restaurants. If you are staying there great, there are things to do but its not a place to go out of your way to get to unless you want to visit a specific restaurant.
If it is the I Ride trolley you are talking about, just get the tickets in the USA.
I too would recommend a Hot Air Balloon Ride as well. We did one in 2005 and it was immense. I think we flew with Orange Blossom Balloons or something like that and they were very good. A airboat ride would be another option as well, not as romantic but it is Florida after all.
As for weather, well you have chosen my favourite time of the year to visit Orlando. It will be warm (mid to late 20's) but nowhere near as humid as summer and more importantly you should miss any major rain. Out of 11 days on our last trip at this time we only had one evening of rain (and it really did rain) but the rest of the time it was blue skies and glorious sun. Crowds should be light as well, I think the longest we queued was 40mins for Men in Black of all rides. The water rides were basically walk on at IOA (btw definitely do Islands of Adventure as well as Universal Studios).
Sounds great, definitely consider the car though. Congrats btw.