Five nights in Istria - where to stay?

Rovinj is probably the obvious answer, though you may find a couple smaller towns they don’t get talked about too much. Rovinj’s swimming areas and beaches are simply the best imo. That’s really the main draw for me. On paper, it’s a small town, but it’s surrounded by resort complexes. When evening rolls around the place is a zoo. Still, the crowds are mostly Italian and Austrian, and they find the old town a lot less interesting than us Americans. You’ll find it retains its charm, and there are surprisingly few tacky souvenir shops and what not. As crowded as it can get, there’s plenty of beach spots you can have to yourself for the most part.

For day trips, Pula and Motovun are within an hour drive.

I wouldn’t suggest this for everyone, but it was an interesting 5 night stay for me. And with a teen who wants beach or pool, it might work for you.
https://maps.app.goo.gl/YTvS3dD4hWxSFuj37?g_st=ic

My friend and I had a small 2 bedroom, 2 bath unit. I reserved at the price level that included swimming pool access, but I walked down to the (pebbly) beach every evening and a couple of times in the afternoon. With a car (and 4 days, we did day trips to Motovun and to Pula. Pula doesn’t get a lot of love on the forum, but I really liked it.

Yes Rovinj for sure. We rented bikes for the day and rode out to the beaches, quick and easy and so many spots to choose from. We found a spot on the the stretch looking back at Old Town and the Church of St. Euphemia, but we could’ve gone further. At least one bar was open nearby, plus people coming by selling snacks. There were giant inflatables in the water for kids and kids at heart to jump and dive off of all day, boats coming through - it was one of the best days of our trip, I could have stayed sun up to sundown!

I loved our stay in Motovun at Hotel Kastel. It wouldn't be a place to spend 5 nights, but you might consider 1. We were on a RS tour and the bus had to drop us off part-way up the hill and we walked the rest of the way.