The magic circle of Snowdonia is the roads linking Porthmadog > Beddgelert > Capel Curig > Betws-Y-Coed > Bleanau Ffestiniog > Porthmadog
The A4085 from Penrhydeudraeth to Beddgelert avoids Porthmadog and has some nice vistas too.
Add also the Llanberis pass.
Depending on how you're getting to Aberystwyth, taking in Lake Vyrnwy goes through some wonderful scenery, also the run south via Rhayader > Abergavenny has some less dramatic, but appealing scenery too.
Unless you're a big fan of 'The Prisoner' I'd also skip PortMerion it's over crowded, over priced and disappointingly shabby.
The Snowdon Mountain Railways is spectacular, but queues can be horribly long and taking photos from the train can be difficult.
If you can, get away from the roads and walk into the hills. Cwm Orthin near Bleanau Ffestiniog is wonderful and worth the short steep walk from where you can park a car. Almost a Snowdonian photographic cliché for good reason. http://www.flickr.com/photos/paulholman/3263031322/in/set-72157613501126730
PM for GPS co-ords if interested.
Sorry never stayed in Hotels or B&Bs up there, I've always camped or self catered. From what I have seen things can be a bit basic, food is also a bit unsophisticated by the standards of other parts of the UK too
The one place I'd recommend eating, and possibly staying, is The Grapes at Maentwrog. Great home cooked food last time I was there.
A note on driving;
Ian might claim it's only 40 miles as the crow flies to Caernarfon from Aberystwyth, (actually it's closer to 50), but by road it's almost 80 and roads in North Wales can be tiresomely slow if you get stuck behind a truck/bus/cyclists/cars towing caravans etc.
The inland road from Machynlleth > Dolgellau > Ffestiniog is a much quicker option to Snowdonia than the coast road which I always avoid.