As to fishing, fantastic fishing in and around here (Yellowstone), Henrys Lake is known for it. One caveat,, because wwe are here where 3 states intersect, plus the Park, you could need up to 4 licenses. You can buy them by the day, just be ready with the right one.
KOA is located just a couple miles outside the Park, there are other campgrounds as well. Grizzly is the nicest, but the most expensive. There are some small ones that are less, being a good place to park the RV and come back to sleep which will save a bit of money. In the park only Fishing Bridgge has hookups. Others are dry camping and ,any with size limits.
Jim & Pat Dick, plus Jolene and I work right inside the Visitor Center in the bookstore at Old Faithful. Kamper Dave and his wife Sue work in the bookstore at Canyon Visitor Center.
Guided tour or not. I know many of the guides and they do a fantastic job, they will know about stopping at places you may just bypass, but do you really need one? Probably not. I have a guide book in the store that is fantastic, plus has a CD with stops. You stop at the point indicated by the map, then play that numbered track for an audio explanation. It also has photos and key as to where it was taken. Guides to best places to see animals, flowers, waterfalls etc. Called Expedition Guide. ($39.95 before discount)
(Not trying to be a commercial, but definitely the best guide we have)
If you like to do a float on a river, head south to Macks Inn. They will rent you a canoe or kayak shuttle you to the put in, and you come back to their place. Beautiful float, might even see moose, plus you can fish the lower part of the river. At the takeout you might want to try their snow cone. (has a scoop of vanilla ice cream at the bottom, really nice and refreshing!)
So much to see and do, so much to enjoy. This is our 5th summer, Jim & Pats 4th summer and we still have lots to do!