Wow, Mandalay Bay is huge. This place would be awesome for bachelor/ette parties.
There are a ton of great dining options - Burger Bar is Huber Keller's gourmet burger joint. My favorite is the Huber Keller Burger. Another great place is Border Grill, which is also my favorite happy hour restaurant in Santa Monica. The margs are perfect - fresh ingredients and not overly sweet. Every single thing on the menu is delicious, and trust me I've tried it all.
Since we've been on a bit of a budget after our Euro adventure we didn't try any of the upscale restaurants, but I heard miX is a must-try. Not only great food, but amazing views of the strip from the top of THE Hotel.
Once you make it out of your hotel casino and onto the street, a few cool touristy things to do are check out the Bellagio fountains and walk around New York, NY.