Don't try to cram too much into that day. The worst mistake you can make is attempting to get "your money's worth" out of your vacation destination. I've seen way too many parents with bulging eyes, throbbing veins, raised voices, and misery carved into their faces, trying desperately to drag their screaming, bored, frustrated children from one site to another, trying to see all of Washington, D.C.'s museums and monuments in a day. I've watched the same thing happen at Disney World. If you're vacationing with small children (I'm taking kids in the first grade and below here), limit your possibilities, enjoy those few items picked to the hilt with your kids, and try to see the place through their eyes as they discover and explore it. You'll be much more relaxed and your kids will be happier. You'll all get the most out of your trip.
By the way, the family we were vacationing with did a brilliant job of keeping things simple and engaging.
*Oh yeah, one other piece of advice. Jettison anything from the day's plan that just doesn't work with those small children. Better to move on than try to "make" a planned activity work that just isn't a fit for your kids. Remind yourself that you'll have more flexibility when your beloved youngsters are a little older and more discerning. Take a deep breath and move on.
Enjoy the last weeks of summer and future vacationing whenever and wherever you go.