hang20,

There are bikes for rent at the Old Faithful area and a bike trail that goes down along the Firehole River. Other than that, you will see some people traveling through Yellowstone with their own road bikes. It's a tough trip! Good for them.
Not sure I would want to be on a bike in a bison herd....

There is no rafting inside the park boundaries, but many opportunities out of Jackson for the Snake River and several out of Madison for the Yellowstone. Just google and you will find all the rafting companies.

I personally haven't done any trips in this area, but there are quite a few old posts of Fodorites who have. Try the search box above and see what pops up.

Happy planning!

You definitely do not want to ride a bike on any of the roads in Yellowstone!!! I did when I worked there for a summer and it was truly taking my life in my hands. No shoulder, RVs, distracted drivers--shudder. The Old Faithful area Dayle mentions would be a great place to bike--and a better way to see those geysers than walking, in my experience. (we didn't rent bikes, we walked and I eyed those bikes with much jealousy ) How old are the kids? I think they have to be 7 or 8 maybe to go on a horseback ride which they have in Yellowstone. Check the rafting company age limits for children. You usually can do float trips with younger kids, while white water trips have a higher age limit. I have rafted the Snake out of Jackson with Barker Ewing many, many times and they are tops.

My kids were 6 and 8 when we went last year and there was enough to see and do that we did ok with out extra activities. Depends on how long you are there etc.

My best friend just came back from Yellowstone/Jackson last night and went rafting with her two 8 year olds who had a blast. They went through the Teton White Water Rafting and wore wet suits because the water was cold (but they were fine with the suits and had a great time)