Interlaken - itinerary help

Luzern to Interlaken Ost is only 1 hour and 49 minutes.
The Tell Pass does not have any use in Interlaken except to get you there and back.

There is the train to the Schynige Platte with views of the Berner Oberland range, but the round trip is 71.20 CHF per person.

You could return to Brienz by boat. That is 32 CHF per person.

At Brienz, there is the Open Air Swiss Museum. https://www.ballenberg.ch/en/

There is also a Sherlock Holmes Museum in Meiringen. Where he supposedly died in a hand-to-hand fight with Professor Moriarty at the nearby Reichenbach Falls. http://www.sherlockholmes.ch/en/Welcome

Interlaken is not a destination. The alpine villages (Wengen, Murren, Gimmelwald, Lauterbrunnen) reached via Interlaken Ost station (change trains and continue) is what you want to see.

You'd probably enjoy your visit to that area more if you venture beyond Interlaken into the Lauterbrunnen Valley. From Interlaken it's very easy to get to Lauterbrunnen, Mürren or Gimmelwald, all by public transit. Would your old parents enjoya cable car ride to the Schilthorn, which has a revolving restaurant and incredible views. It was made famous in the James Bond movie On Her Majesty's Secret Service a number of years ago. It' at about 10,000 feet so if none of you have any "altitude issues", it's a nice trip (albeit a bit expensive).

I agree that you need to go beyond Interlaken. I also agree that your parents could enjoy the trip up to Schilthorn especially if skies were clear. The James Bond connection makes it quite fun. The Lauterbrunnen valley has a few stunning waterfalls that might interest you. Have you looked at the Swiss Travel Pass? Schilthorn is included in that. If you decide not to do Schilthorn, you still could consider a cable car to Gimmewald (Where Heidi was filmed) and Murren for stunning views. You would be on the train quite a while that day. Have you thought about perhaps staying a night in Berner Oberland? In any case, that train ride is very pretty and that by itself may be great fun for your parents.

I agree with the other replies. If you are based in Luzern, there is NO reason to go to Interlaken itself for a day. The town itself is not nearly as nice as Luzern, and both are on lakes, so you don't need to go to leave Luzern to take a lake cruise.

If you are going to the Lauterbrunnen Valley, that's great, and you need not go up the Jungfraujoch to make it a great trip. If the Schilthorn is too expensive or time consuming, you can go up the Allmendhubel, which is a cheaper and shorter ride that still gives great views of the three peaks. Even a walk along the valley floor is worthwhile, and there's a bus all along it, so you can walk as far as you like and then take the bus back when you get tired (this would be my top recommendation for this area for anyone with potential stamina issues).

If you are interested in the Ballenberg Open Air Museum (old houses and structures from various parts of Switzerland), that's another worthwhile daytrip from Luzern. You take the train from Luzern to Brienz, then a bus from there right to the gate.

While in Luzern, in addition to Mt. Pilatus, you can also go up Mt. Rigi. From Luzern to either Riti or Pilatus, you will cruise on Lake Luzern, which is very nice too. Just be sure that if you're going to Rigi, you take the boat to and from Vitznau, NOT Weggis. Between the train station and boat dock in Vitznau, it's a short and nearly level walk. Between the train station and the boat dock in Weggis, it's a VERY steep and long walk (took me 10 minutes downhill; would have taken me 20 minutes, at least, uphill).

I have done the trip to Mt. Rigi from Luzern and it's a wonderful suggestion. The boat ride is lovely and the cog train is fun with lovely views. The stops are to level ground and even at the summit, there is level ground to walk on for views. There is some kind of small coffee shop/restaurant at the top. Sorry, I didn't go in. The boat dock is convenient and across from the train station.