Reichenbach Falls and Aare Gorge hiking difficulty

Assuming that by "Swiss Rail Pass" you mean a Swiss Travel Pass that will be valid on that day, here is a list of museums covered, in alphabetical order by city, as a PDF: http://dev.museumspass.ch/wp-content/uploads/sites/5/2017/05/Falterli.pdf

and a PDF by canton: http://dev.museumspass.ch/wp-content/uploads/sites/5/2017/05/Museumspass-Booklet-04_2017-Webversion3.pdf

Yes, the Sherlock Holmes Museum in Meiringen is covered (as is the Musée Sherlock Holmes in Lucens - I had not previously heard of either the town or the museum, but I see it's not too far from Lausanne if you'll be in that area).

The Aare Gorge website does not show any reduction, much less free entry, for the Swiss Museum Pass https://www.aareschlucht.ch/en/Info/Prices#.WdhKXHZryUk. Nor is it listed on the Museum Pass PDF list I linked above. It never hurts to ask, but don't count on anything.

The hike up to Reichenbach falls without the funicular is quite steep (total altitude increase circa 1200 feet) and I believe the roundtrip would take around 3 hours. If you consider yourself 'pretty sedentary' I think it might be a bit much. One option would be to take the Postbus from Meiringen train station to the stop "Zwirgi" (Route 164, about a 15 minute ride). This route runs until the end of October, but check the timetables for the exact schedule https://www.postauto.ch/en/timetable - it is normally about 1x an hour. There is a restaurant by the stop (closed Tuesdays) called Gasthaus Zwirgi where you can get lunch/coffee/etc if you want. From Zwirgi you can take a short 5-15 minute walk to the falls (or even walk all the way back to Meiringen, downhill).

Walking to and through the Aare gorge is mostly flat. It's about a 1 hr walk from Meiringen to the east entrance, 1 hr 40 m to the west entrance. You can always opt for the train to/from the entrance if you don't feel like walking the whole roundtrip. It takes about 45 minutes to walk through the whole gorge.

Thank you! That's exactly the detail I was looking for.

The Sherlock Holmes Museum in Meiringen is delightful and is free when traveling on a Swiss Travel Pass. It explains the history tie-in with Meiringen.
The Aareschlucht (Aare river gorge cliff walk) is spectacular and very easy. It's not covered with a Swiss Travel Pass, but admission is only 8.50 Swiss Francs. I've gotten a 2.50 discount with a Meiringen visitor's card when staying in Meiringen, but since you're staying in Gimmelwald that won't apply. To get there, when you take the train from Interlaken Ost to Meiringen, and get off at the Meiringen station, transfer to the little Innertkirchen train (I think its little track was called Gleiss (Track) 13, although there are only about 2 other tracks there for Meiringen!) Take the Innertkirchen train (runs through October 29th) to the Aareschlucht Ost train station (Press the "an verlangen" [on request] red button when it's announced) You'll be inside a mountain tunnel. You'll exit the train and come out a door in the cliff of a mountain glacier moraine! Walk across the suspension bridge over the Aare river and look across the river to the other side to see the cliff walk. There is a nice path with steps at the other side of the walking bridge that takes you up to the parking lot and the entrance to the Aareschlucht cliff walk. Once you walk down the stairs in the cliff walk, the rest of the cliff walk is flat and follows the rushing river.
When you get to the other end of the walk, you'll exit at Aareschlucht West. Walk through the souvenir shop to get out. Then walk down the main driveway/road to either wait for the train back to Meiringen, or, since at that point you're closer to Meiringen, walk along Sandstrasse (SandRoad) through the neighborhood of chalets, over the Alpbach bridge, and into the shopping area of Meiringen, along Rudenz street which becomes Bahnhofstrasse, and continue walking on Bahnhofstrasse. Turn left at Hotel Victoria and you'll see the train station behind that hotel.

Thanks to all for the information. We went to Meiringen and the Holmes Museum which was nice. Since the funicular was closed, we hired a taxi to take us to the upper entrance to the falls and hiked down. Halfway there, the trail was blocked off. Some ladies came up from lower down and told us not to go around the barrier because they had put up a wall lower down so you really couldn't access the falls. It would have been nice if the trail had been blocked at the top! We climbed back up and luckily caught the post bus back to the train station. Very disappointing but then travel is always an adventure.

Thanks to all for the information. We went to Meiringen and the Holmes Museum which was nice. Since the funicular was closed, we hired a taxi to take us to the upper entrance to the falls and hiked down. Halfway there, the trail was blocked off. Some ladies came up from lower down and told us not to go around the barrier because they had put up a wall lower down so you really couldn't access the falls. It would have been nice if the trail had been blocked at the top! We climbed back up and luckily caught the post bus back to the train station. Very disappointing but then travel is always an adventure.