In Excursions from Vegas I touched upon the various national and other parks you can visit within reasonable travelling distance of Vegas. There is now an invaluable resource that brings even more of this information together in one place – Find Your Park.
There are effectively ten significant parks and recreation areas within one hour of the city and I don’t mean just that bit of scabby grass between the buildings that’s used as a giant dog toilet!
If you feel up to a five hour drive there are many more wonders including the Rainbow Bridge and Petrified Forest parks (I have one of my pictures from the latter as my PC wallpaper).
Please try Find Your Park. This is a unusually good site for the National Parks Service.