What town/area are you staying in? Are you staying in Summersville? Are you staying in 1 place for the week or traveling around? The reason I ask is because I don't think all the things you are doing are right in Summersville (I could be wrong). However, they are not far away. Knowing where your base is would help with the "don't miss" things suggestions.
The Greenbrier river is gorgeous.
If you go to Hawk's Nest or near the town of Ansted, be sure to check out the Mystery Hole. It is a cheesy looking little tourist spot but it is awesome - as it says " where the Law of Gravity seems to have gone berserk ". http://www.mysteryhole.com/contactus.html
Goto http://www.wvtourism.com/planner.aspx?pgID=57 to help plan your trip. You will need to click on Summersville and also on Fayetteville (I am almost positive the Fayetteville area is where you will be whitewater rafting). Summersville is in the Mountain Lakes Region and Fayetteville is in the New River/Greenbrier Valley Region.
Also check out
1. summersvillecvb.com/summersvilleLakeRecreati… 2. http://www.visitfayettevillewv.com/
3. Check out the State parks at http://www.wvstateparks.com/map.html Hawks Nest would be the closest to you. http://www.hawksnestsp.com/recreation.htm
You taught me something new - I've lived in WV for 25 years and never knew there was decent snorkeling anywhere in WV. So I checked out the Summersville website and it does say "Scuba diving and snorkeling are also very popular in this oligotrophic lake. Bubbles Cove and Long Point are favorite spots to snorkel and scuba in what Skin Diver magazine refers to as “The Little Bahamas of the east.”