We love the Denver Downtown Aquarium. I have been to several aquariums, and this one is by far the best I've seen. If we lived less than nine hours from it, we would definitely get a membership and visit more often than our once a year. (No one is paying me to say this, we just really like it.) It's a little pricey- Adults- 14.95 each, Children 2-11- 9.25 each, plus you have to pay $7 for parking. But there is a printable coupon at this link for $5 off a child's pass with the purchase of an adult. Although it can be expensive, I think that it is worth the price. There's also an opportunity to swim with the fish, or if you're scuba certified with the sharks. (My kids aren't old enough, so we haven't tried these.) If you go on a Friday night (we didn't) the kids get in for $5 each (no coupon required) plus free face-painting, animal shows, scavenger hunts, prizes and free parking. Even without these extra things we had a great time. My two-year-old kept squealing "There are lotsa fishies!" They have some enormous fish, plus a large variety. Even without swimming with them, you can get some great views of the sharks. And for some reason there is a tiger exhibit in the middle of the aquarium. This place is awesome. My pictures don't do it justice, but here they are.
Purple, Blue & Red Starfish
Look at that enormous fish! (My shot of their 350 lb grouper didn't turn out.)
This one makes my daughter look like "Cousin It", but I included it so you can see how close you can get to the fish.