You should have a nice trip - I'm envious!
I'd recommend having a look at one or more of the enclos paroissial
- I'm particularly fond of the carvings at Saint Thegonnec - they are very humorous and well-preserved - they look like they were carved yesterday. More-or-less due south of Roscoff and so easy to get to on Day 1 - the villages featuring the parish closes are a bit touristy, but that means more-choice-of-restuarants when it comes to lunch
Huelgoat is typically Breton, wooded and on the river Argent. A good place to stop for a night, I imagine. I've only ever been through it on my way to somewhere else so can't recommend a particular campsite.
Whizzing east a few miles... I like the harbour town of Paimpol, based around a large double harbour, home to a biannual music festival (first week in August...). There's a fairly big cheap camping municipal at Kerity s.e. of Paimpol, 5 min.s stroll from the coast. Paimpol is a good base if you fancy a trip to the small island of Ile de Brehat (which does have a camp site but I'm not sure if it will be open in May). The island is lovely! Paimpol itself is quite posh, with lots of bars, restaurants and shops. And if you like chuff-chuffs, it's the destination for the restored steam-train which you can take to get you down as far as Guingamp, I think. Le train vapeur
follows the valley of the river Trieux and it's a lovely trip! Toot Toot! http://www.vapeurdutrieux.com
Guingamp is a decent sized town, and probably worth half a day, although I can't think of any special reason to go there other than the Saturday market... others may know of Guingamp's hidden attractions!
Dropping down further south, I like St Brieuc. I avoided it for years thinking it's a modern French town, complete with blocks of flats and a nasty white-van infested camping municipal, which it is... but the centre of town is great and I think it'd be worth an afternoon if you don't mind towns!
If you like sea-food, you're obviously in the right place but if it ties in with your travel plans consider 'over-shooting' St Malo and going to Cancale for lunch. The oyster beds are just out in the bay and you can sit in one of the many cafés (along with all the other tourists... the place is popular
) and watch your lunch come out of the funny aluminium skiff-thingies they use and straight onto your plate. 'Cos of all the competition, the prices are rock-bottom but the quality is up the top end
If you have never 'got' oysters give half-a-dozen a try here and you'll find out what all the fuss is about!