Actually, Lauterbrunnen is the real horror: thick fog (whereas there is bright sun at Grindelwald, from mid morning until mid afternoon). But even if there would be no fog or clouds at Lauterbrunnen, the sun would be behind the mountains most of the day and just appear towards noon in order to disappear in early afternoon.
But between March and early October, this problem doesn't exist.

BTW: mrt didn't come back during more than 4 1/2 days

Thanks for all of your responses. Of the three towns I mentioned Wengen seems to be the clear choice for a summer trip. I will also look into Interlaken as a base with day trips to the other towns

mrt - we spent 4 nights in interlaken and 3 in Wengen for our recent trip. We wanted to be able to take boat trips on the lakes, and make the odd trip to a city like Bern, and then to be able to enjoy a few days actually up in the mountains.

For what you want, it sounds to me as if Wengen would be the better choice - you can still day trip if you want to [you will have plenty of day light after all and it doesn't take very long to get from Wengen to Interlaken Ost where the trains to bern etc start from] and you will be in the ideal place to enjoy all the walks and scenery you could possibly want.

If you only have 3-4 days there don't waste time relocating between Interlaken and Wengen - stay in one base. annhig was there for a longer time and strategically planned her trips from Interlaken and Wengen.

En route to Zurich from Interlaken you could easily visit Lucerne for a few hours - take the scenic route Interlaken-Lucerne via the Brunig Pass rail route - part of the Golden Pass scenic train route.

In fact depending on hour of plane you could easily stay the night before in Lucerne - a much smaller place than Zurich and one that is so so much more romantic in its lakeside setting and lake front hotels - just little over hour Lucerne to Zurich Airport by frequent rail connections.