One of Vegas's not so well known gems is the Railroad Tunnel Trail. I've walked it, rode a mountain bike on it, and no matter how I choose to experience it I always enjoy the experience. Bring your camera, you'll kick yourself if you don't.
CRITICAL NOTE: There is NO DRINKING WATER along this trail. Be prepared, bring plenty of water with you!
This is a former railroad grade...so you're looking at easy elevation changes except when you get close to the dam.
One other local's note. The closer you get to the dam, the hotter / windier it gets. Like as I write this there's a wind advisory up and it's for...the Colorado River from Lake Mead downstream and that puts the trail right in the middle of the typical wind advisories.
There are park benches at strategic spots that are great if you want to have a seat and look out over the lake. Be prepared for the thump thump of the tourist helicopters buzzing over head though. I'll take the noise of the helos over a bunch of slot machines any day.