Definitely take the time to walk through the Venetian, and the Bellagio. You will understand why when you get there. The show at Treasure Island is fun! The Bellagio has a very good and reasonably priced restaurant. I spent time in all of these and never gambled a penny.
Hoover Dam is worth the trip.
Valley of Fire State Park is also worth the short drive--worth a day trip at least.
Old Las Vegas Mormon Fort State Park is interesting, as is Spring Mountain Ranch SP.
Lots of info about these on the Nevada SP website:
http://parks.nv.gov/parks