I have hiked Rifle Falls in the past -- it's an easy, quick hike, and the caves are fun to explore (if your dog is as scared of caves as mine turned out to be, bringing a dog may impede your ability to explore the caves).
But while you're in the area, go up the road a little further and visit the hatchery! It is free to tour, there are some educational signs posted in their visitor center, and you can see some enormous trout up close. (No fishing allowed.)
If you go between 7:30am-4pm, ask an employee for a tour or to answer questions. Our guide was interesting and not at all put off by having to take a break and talk to us. They do have a donation fish head (see picture), and we donated because of how much time he spent answering our questions.
There are coin machines where you can buy food to feed the fish, or you can just get some for free from the person you talk to. It is fun to watch the fish behavior when you feed different amounts or different size fish. Don't ruin all the fun by dumping your food all at once.
All of the hatchery except for the indoor portion (which was not really in use when we went) is wheelchair accessible, and they have special nearby handicapped parking. Be aware that the surface is bumpy asphalt, so I recommend a power wheelchair or a family member to push you.
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