The Mirage is an older hotel, but it's always been one of my favorites. There's something about its quasi-tropicalness that makes me feel as if I've been transported to a jungle paradise with flashing lights and bells from the casino floor. I've never won very much here, but I usually at least can play long enough to have fun and on my last visit, I discovered the new Plants Vs. Zombies slot machines which were adorable.
Especially if you have kids, the Dolphin Exhibit is well worth a visit, and we've come here many a time over the years. The Secret Garden is not as well designed in my opinion, but is worth a visit if you've never been. They're currently hosting Cirque du Soleil's LOVE show, which is incredible and a must-see, and the theater is really well designed (although it reflects the Beatles' and 60s England which makes it a bit incongruous with the tropical waterfall theme of the rest of the resort). On the outside, the (free!) volcano show is worth catching if you're in the area.