The things that travel4fun lists are wonderful to do for sure, but I agree with Wanderweg, it would be quite a lot to do all that in one day.
When planning how long a journey will take, the train schedule is very exact…although there are occasional disruptions, but all things working as usual Bern - Wengen will take 1h 38 min. Once you arrive the timing is not as easy to figure out.
If it is your first time in Wengen, or even your 100th time, it is so beautiful that you will likely take a moment to look around, take a few photos, see the person with their ice cream cart and suddenly realize you need some ice cream...
Now walk to the Männlichen gondola station. Not far, but perhaps a bit more looking as you go, look in a shop or two, take another photo because you just can't believe the beauty. Now you have noticed the Jungfrau, quickly snap a few photos of that...
It would be a pretty unique situation indeed if you were able to arrive at the gondola, walk right on and depart immediately. I have been there when there was no line-up and could walk right on…but I still had to wait about 15 minutes as the gondola filled up and was ready to leave.
So, 1h 55min to get from Bern to the top of Männlichen is pretty optimistic. Once up there you will also want to take time to look around, of course….and on and on it will go. As the day progresses things will get more busy and you can sometimes wait in lines for a long time to get onto the various gondolas. This is always something to take into account when planning.
There is so much to see that you will never get it all in, to just relax in that and go with what seems the most interesting to you, adding on as time allows, seems like the best way to go about it, in my opinion.