OK, that helps me to help you.
I figured the time in Interlaken is for kayaking, but I didn’t mean exactly which lake for the kayaking; just wondered whether you meant river kayaking or lake kayaking. There is some serious whitewater kayaking on the river between Interlaken and Lauterbrunnen, or pleasant flat water kayaking on the two lakes right by Interlaken. This website shows photos of the options on Lake Thun and Lake Brienz:
https://www.interlaken.ch/en/experiences/lakes-waterfalls/water-activities/kayaking
Of those, I personally would much prefer the Lake Brienz options with the High Tide Kayak company. They offer true sea kayaks which are a pleasure to paddle. The “sit-on-tops” shown in the Lake Thun photo are heavy, clunky, and not good for much but sitting on (in my view). And the kayak trip on Lake Brienz to Giessbach looks wonderful ( I have always wanted to visit that waterfall). There are more photos in this blog:
https://www.travelhackergirl.com/kayaking-interlaken-to-giessbach-falls-kayak-rental-with-hightide-kayak-school/
I think this could be a nice experience to include, and a good reason to spend a night or two in Interlaken. The rest of your time is best spent as you have planned, actually “in” the mountains, in places like Grindelwald and Wengen.
I find the views looking up at the mountains from the town of Grindelwald to be lovely. I hope you have found a hotel where you can enjoy that view even from your room. Grindelwald offers good access to a number of easy hikes, including the walk to the beautiful Bachalpsee from the top of the First gondola:
https://swissfamilyfun.com/grindelwald-first-bachseealp/
That is a short hike ( maybe 2 hours at most), so if it is a nice day you could add the 7 km “Marmot trail” from the top of the First gondola over to Grosse Scheidegg:
https://jungfrauregion.swiss/en/summer/tracks/marmot-nature-trail-first-grosse-scheidegg-no-17/
Then a Postbus from the Grosse Scheidegg restaurant back to Grindelwald.
Another beautiful hiking area you can access from Grindelwald would be Schynige Platte, which you reach by train from Wilderswil, a short distance from Grindelwald. This is between Interlaken and Grindelwald, so you can also do this from Interlaken). A steep cogwheel train takes you up from Wilderswil to Schynige Platte, where you will find a hotel and restaurant, a nice alpine botanical garden and a choice of several short hikes with beautiful views over Lake Brienz on one side and the Grindelwald Valley on the other. This blog has a nice overview of the walks:
https://fullsuitcase.com/schynige-platte-hikes/
Or right from Grindelwald, you can ride the Pfingstegg lift and follow the trail above the glacier canyon to a mountain hut (Baregg) with more great views. This is an easy walk on a nearly level trail, but it does go along a cliff edge—-not so close as to be at all dangerous, but if you are troubled by heights and steep drop offs (as I I am) this might not be so easy. Still, it is worth going up there, even if you choose not to walk all the way to the hut.
https://swissfamilyfun.com/pfingstegg-baregg/
I keep coming back to that “Swiss Family fun” blog because they do a great job of describing easy hikes, with photos. Note that there is a link to more such hikes in the Jungfrau region on that page.
Finally, there is the famous Männlichen to Kleine Scheidegg hike, which you can do from Grindelwald or from Wengen. But since you actually plan more time at Wengen than Grindelwald, you will probably want to save this hike to do from that side. The ride on the cablecar up to Männlichen is pretty thrilling.