I can’t believe that no-one has mentioned the A5 through the Ogwen valley. There are a couple of good campsites on the road, as well as a YHA.
Not to mention Tryfan (the most awesome scramble in Wales).
Taking the A470 down from Betws past the Trawsfynydd reservoir (some easy walking and cycling around it) with views of the Rhinog mtns inthe background (not much easy walking there though).
Also Barmouth is a cool place to visit, take the A496. Some great views of the Lynn peninsular (though much better view can be had from Rhinog Fach especially at dusk/dawn).
Or take the A487 as mentioned in other post, there may be some noisy jets screaming about on the Mach Loop.
St Davids is great (walk the coast path) but you’ll be struggling with time unless you love driving that much!
Have fun Wales is awesome!!!