It is $10 per car entry that is good for a week. There are plenty of things to do here. Lots of hiking trails. You can swim in some areas marked but the water temp is little cold. You can kayak, scuba dive, biking, camping, fishing etc.
Lake Mead offers campground for RV's and tents. Camping is 10$ a night. If you are in a mood for cruise, take The Desert Princess cruise located at the marina. There are 3 different trips offered throughout the day. The cruise takes you on a tour of the hoover dam and the lake.
You will sometimes see wildlife, I have seen several big horn sheep. You can also check out the lost city museum for $5, it contains a lot relics and info about the area.
Definitely a place to visit if you are tired of the Strip.