Depends on your route but here’s a few.
Pembrokeshire is my home county so I can give better ideas if I know you’re approximate arrival times and routing.
Dolgellau is a pleasant town with reasonable facilities.
King Arthur’s labrynth, between Dolgellau and Machynlleth is good for a toilet stop and has a cafe.
Machynlleth – another town which will have coffee shops etc. or call in to the Centre For Alternative Technology.
Ynys-Hir, just before Furnace , has toilets etc. and is good for a picnic.
Aberystwyth has all the major supermarkets.
Aberaeron has a good fish and chip shop (on the side road towards the harbour).
Cardigan – loads of places during the day and also a Tescos.
Crymych – a nice cafe during the day – toilets in Crymych are shut at the moment but you could use the leisure centres I’m sure.
Efailwen – Cafe Beca.
Clunderwen – toilets and a park.
Narberth – cafes, toilets etc.
Rich.