Hope the weather picks up for you,we were in Barmouth for a week in June.....great places for food just down past the bridge heading into town.If you're S/C you can get supplies delivered from Co-op for a spend of £25 I recall......easily done with a few bottles of vino........we only used this service as there was no vehicular access to the place we'd rented,well up the hill behind the town centre.
Had good drives/days out in Port Merion,Snowdonia and headed south for a drive along the coast.
Enjoy your trip