Hi Mark, great idea! Definitely off the beaten path, you may not get too many responses here, but I've been to the area a few times, so I'll share what I know.
Firstly, look into this site https://rowermevo.pl/en/, it's a bike sharing service that operates 100s of stations along Poland's Baltic coast. Not sure how intense of a biking experience you're looking for, but in addition to Hel, you could also consider these areas for biking activities:
Sopot: traditional Polish beach holiday destination, nice small seaside esplanade and pier.
Gdynia: the more built up cousin to Sopot, has a fairly longer esplanade that snakes along the Baltic coast. You can start here Przystanek MEVO (bike sharing station), and then bike up to the port area which has a number of museums and restaurants.
Słowiński National Park: Not sure if you can do much biking here, but it's a very unique stretch of Baltic landscape that's listed as a UNESCO World Heritage Site.
Masurian Lakes District: As an alternate multi-day side trip, just inland from the Baltic coast, this corner of Poland was once part of the historic region of Prussia. It's dotted with gingerbread medieval towns, Teutonic Knights castles, and other curiosities like Hitler's Wolf's Lair Bunker, all strung together by 1000s of waterways and small lakes. Very popular holiday destination for local Poles looking to do some cycling or canoeing. Here is a recent article on the Masurian Lakes District from The Guardian: https://www.theguardian.com/travel/2019/jul/02/poland-lake-district-rivers-canoe-kayak-cycling