One of the things I hear folks complain about the Mandalay Bay resort is that it's too far from the action. On the other side of the coin, you could argue that at the southern end of the strip, you never have to wonder which direction to go - just head north. But seriously, if you ever wanted to cut down some of your walking time toward the centre of the strip, just catch the (free) tram to Excalibur.
But while you're at Mandalay Bay, there's lots to do. The Shark Reef is always fun. If you've never been to a large aquarium, this will impress you. At $15.95 per person, you might think twice, but if you're staying at the resort you might get in for $5 per person instead.
I think the major attraction is the Beach. With a wave pool, lazy river (our favourite) and other lagoon pools, beach sand almost everywhere plus service right to your chair, you can't go wrong. If you plan on going to the beach with 2 or more people and you want chairs together, better get there before 11am. On busy days, most of the chairs are full by noon. To best enjoy the lazy river, you'll want an inner tube. The Beach rents them for an astronomical price - I won't even tell you - your brain will bleed. Better to buy a cheap one outright at $16 each.
X marks the spot we claimed at 10:30am. Y is where the topless beach is, if you're into that sort of thing (it's not free).