Hoover Dam, close to Vegas is an interesting half-day trip, particularly if you have kids. You can go on a tour under it through tunnels, which they will love. Obviously not suitable for claustrophobics but the tour guides are very entertaining. Bryce Canyon was also a good day trip. Take lunch with you because the only place for lunch there was a tad expensive.
If you are going to have a look at a lot of National Forests/Parks look into getting a pass. You'll save a lot of money.
