The coast, do as much walking / biking there as possible, imo its the star of the show. The hills inland in the Parc itself are worth a look and maybe a ride – have only driven through between Morlaix and Port La Foret. If the weather is spicy a trip to see the famous lighthouse at Raz de Seine maybe worth it.
As its out of season finding things open may be a struggle, Cameret is ok, Dournanez is better for boats, old inner harbour should have some wooden boats and a few bars / restaurants alongside. May find something with some atmosphere for RBS 6N rugby on Sat (France v Italy or later Eng v Scotland)
I have sailed all around the coast quite a bit, stunning area from the water. Only really know the harbours that far West.
Not sure if any of these routes are any good – click on the area
https://www.utagawavtt.com/map/carte-generale-topos-vtt
Have fun, post some photos