I say Yellowstone, but only because I haven't ever been there and have been to Vegas lots of times.
The Deep Space Nine is a great deal since it costs about $15pp to get into the star trek experience, and quarks bar is not cheap (and is a fun meal). The only downside is that the LV Hilton (which has Star Trek and Quarks) is at the end of the strip and not close to any other casinos. You can walk to a few places, but not much, particularly during the summer. You can take the trolley ($1.25?) and see the sites.
The lazy river is at Mandalay Bay - they also have a wave pool. It's close to Excalibur (and in the same hotel chain). You just take a tram (think monorail) to Mandalay from Excalibur. As someone said, you probably won't be able to see the pool without a room key. They do check at Mandalay.
The new resorts have better smoke filtering systems BUT if you sit anywhere near a smoker, you're in trouble!
If you have any other questions, feel free to PM me. We go 1-2 times a year.
Lisa