Here are some suggestions:
1. Best hikes - most of the really good accessible hikes are closer to Buena Vista. One of my favorites is Ptarmigan Lake (Drive W on Main St. out of Buena Vista towards cottonwood pass. The trailhead for Ptarmigan lake will be on your left side (South) after the Avalance Trailhead and Denny Creek). There is a lake about 2 hrs up the hike where you can camp overnight if you would like to or you can continue up to the top where there are more lakes and take a dip in the cold alpine water - quite refreshing. It is at high altitude so bring layers for this hike.
2. 14er I would suggest Yale. It is beautiful and one of the shorter hikes. This way you will have plenty of time to do other things in the area. Any of the views from the top are wonderful. Check out this website for more details: Buena Vista Chamber of Commerce - 'Buena Vista Chamber of Commerce News and Events' Hiking In Buena Vista, Colorado
3. Hiking into the #'s: I would suggest starting at the put-in and then you don't have to hike in, however if you do want to hike you will have to drive from Main St. in Buena Vista and turn North onto Colorado. Follow Colorado out and there are numerous access points below #5. If you want to go further upstream it gets more tricky. Or you could start at the put-in and take out before # 5 or at one of the locations along the road there, depending on what you want to do. I suggest stopping by CKS (on Main St.) and ask them for suggestions.
4. Hiking on the tracks: No trains running there.
5. Flat Water suggestions: Again, go to CKS they are the resident experts on SUP locations and rentals, however I would suggest Cottonwood Lake (again up Cottonwood Pass) The water is cold but the views are spectacular and it is a fairly large lake.
6. Places to eat:
BV: Asian Palette - great thai and sushi or Eddie Line - wood fired pizza and brewery
Salida: Laughing Ladies ($$ but awesome food), Currents, or Amicas - the pizza place you were thinking of, and yes, they are still open and serving their delicious Chili beer.
7. Suggestions for your days off: Rent a bike from BV Cruisers and cruise around Buena Vista. We have 10 wonderful cruiser bikes and lots of other great suggestions of places to see and things to do. Let me know if you are interested. firstname.lastname@example.org
Good Luck, Sounds like a great vacation!