Personally - I'd stick closer to home and and maybe visit places that are laid back. You said you'd be frustrated to be in an exciting locale and not be able to do all the things you used to, so why not hold the dream destination for when you have a babysitter and find something new?

Key West - yes there is scuba diving, but why not just stick to the awesome snorkeling? Warm, not too exotic but a little removed from your everyday life.

How about some city tours - Boston, Washington, DC, NYC? Plenty to do on foot in those cities so you can keep moving and not just veg on a beach chair. Newport RI for that matter is beautiful and has some awesome scenery.

Why not take a train trip through Canada? Hit Las Vegas and do some of the easy walks/hikes in Red Rock Canyon and Valley of Fire. Heck you could even go to Bryce and Zion in Utah and do plenty there. You may not do the most exerting hikes, but if you are in good shape (and it seems you probably are considering your past vacation activities) you can modify alot of what you'd do pre-pregnancy and make it safe for your current condition.

Just remember, drink lots of water and if you do fly distances, make sure you move around as much as possible.

And happy travels.