M.F.
In our experience with 2 kids under 4, and having taken vacations when we had just one, then again with both...low-key vacations have been the best and most relaxing. Anything more "hyped" doesn't feel like a get-away -- it feels more like work, the kids are cranky, and we're exhausted when we get back home! If you're up for a "big" trip, I'd suggest going when you have a lot of time to spend at your destination.
It typically takes our kids about 2 days to assimilate to the new surroundings (this is the travel day, plus one more full day). By the third day they are getting back to regular napping/sleeping/eating routines and we can begin to really enjoy ourselves.
That said, our favorite trips have been little road trips to places with kid-friendly cabins or B & Bs, where the kids can do a lot of running around. Amenities we look for are grassy areas, playgrounds, pool, pony rides, and rooms with at least a fridge if not a kitchenette. Sight-seeing is out unless it's a trip to a kiddie museum or some place where they can stretch their legs and/or interact with something. Sure, when my daughter was 8 weeks old at the San Diego Zoo, and again at 6 months old at Disney World, she was perfectly content spending hours in a stroller...but that was over as soon as she became mobile.
When our first was a year old, we took a little road trip to New Braunfels and stayed at Stage Coach Stop B & B. The Inn owner was wonderful, breakfast was phenomenal, and the place is right between San Marcos and New Braunfels, so we didn't have far to drive to some of the lovely parks and areas down there. We really just puttered around a lot, but it was very relaxing.
Last year we went to Guadalupe River Ranch in Greune, TX and had an even better time because we never had to leave the ranch! The sand volleyball pit served as a giant sandbox for the kids, the swing set was right near our cabin, the pool was nice, we had fun splashing in the river and taking little mini "hikes", roasting marshmellows, horseback riding, and feeding crackers to the deer that roam around the property. For meals, there is a restaurant on site as well as a gift shop with various snacks and ice cream. We drove into town one night for dinner and wandered around antique shops. The drive down there was nice too. We left early and planned to take stops as needed (which is often with little ones). We stopped off for lunch in a small town then played at a local park, drove while the kids napped, then stopped off in Lukenbach for dinner and live music.
My daughter (3 1/2) still talks about our vacation and has recently been asking to go again!
Yes, we are dying to pack them up and head to Europe to visit friends, but it will be a couple more years before we're brave enough for that trip. I cringe at the thought of my 21-month old on a plane for more than an hour.