You could try the southern section of the Rhinogs. Start at Barmouth and get up on the ridge to the NE of the town. Since it's mainly ridge walking, assuming the weather is good, you'll get good views for most of the walk (including the sea to the west).
In a couple of days you could get to Cwm Bychan, although once you get to the two Rhinog mountains (Fach and Fawr), the terrain starts to get a bit difficult. You don't have to do the mountains however, and there's a path that turns into a road from between the two mountains that leads west to Llanbedr, and if you get to Cwm Bychan you can head west to Harlech. There's a regular bus service that would get you back to Barmouth from either town if you needed to.
Unless your're feeling really fit, I wouldn't suggest going north of Cwm Bychan.
I'm thinking of doing this myself later in the summer, because although I've already done it before, it is a good walk.
S.