That should give you lots of time. You can travel PM 21st, have all of 22nd, and somepart of 23rd. I'd tend to leave a few hours extra in the schedule to get the airport, just in case.
In that time you could travel quite a bit.
Depending on routing and timing choices, you could incorporate a scenic train ride.
Since Luzern is only an hour or so from the Zurich airport, more or less, I'd tend to finish up at Luzern. One train each hour at xx35 does not require changing at Zürich main station. Others do. (Not to worry if you miss that connection; there are 5 or more trains per hour from Zürich main staion to the airport).
You could spend all your time in the Luzern area, including Titlis.
Or, you would have time to go Interlaken or Lauterbrunnen or Wengen, stay the night, get up in the morning and visit Jungfraujoch or Schilthorn or otherwise explore the area, and travel to Luzern in the afternoon or evening.
Note the comments under "Where to Stay."
You may go via Bern or via the more scenic route over the Brunig Pass. The route over the Brunig Pass is part of the Golden Pass routes and would go via Meiringen and Brunig. It is also the fastest route, so the timetable shows it automatically. Half the routes through Bern, for which you must specify "Bern" as "via" are 10 minutes longer.
On the 23rd you could easily visit Titlis and look around Luzern a bit afterward, then go to the airport at 1700 or 1800. Later is theoretically possible , but I tend to be cautious.
The train is the best way to travel. Use the timetable to determine times. This page has a lot of useful information and will help you use the timetable effectively:
After you are used to it, you can go directly to :
Here is information on the Golden Pass routes:
I use this map when I use the timetable:
If you zoom in far enough and turn on transport stations in the menu, you can see these details:
Mouse over a station icon to get its official name, for use in the time table.
The route over the Brunig to Luzern passes through towns that would also be on the route back out from Luzern to Engelberg ( Titlis), such as Hergiswil. Although staying in Luzern is really nice....nice town to wander in....you could save time by staying in Stans or perhaps Sarnen when you come from the Jungfrau region.. For Stans, you'd change at Hergiswil.
Check the map:
On the way from Interlaken to