Beaumaris: park on the green on the front, I think it’s now £5 for the day & there’s plenty. If you want to do the Jail get the duo tickets that let you into the old Courthouse on the square opposite the castle. The castle is amazing and if you’re there in August then the Medieval Fayre over the Bank Holiday is fantastic - it’s full of all sorts, as well as a mini medieval village in the inner bailey which will have a bowyer, barber surgeon, forester & quite a few others.
Don’t get chips in Beau, use Sea Wave in Menai Bridge, far better and cheaper. And you can ring the orders in.
Afternoon tea in the Bulkeley is good & they often have craft fairs there on a Sunday; there’s often a craft fair outside the library on a Sunday too.
Wal in Caernarfon is fantastic, the medieval walls in the back are part of the old town.
Harlech is great; there’s a lovely little ice cream shop just up the hill from the castle & a deli / pie shop opposite that does amazing pasties as does the one at the top of the hill.
Portmeirion is pretty but definitely overrated; Barmouth is quite a drive from Caernarfon even with the new bypasses and is basically a long street with the admittedly nice beach and fair behind it. There’s more to see in Harlech / Criccieth tbh. If you want a run down the Llŷn Aberdaron is pretty and has a nice beach but parking can be an issue. Dinas Dinlle has the (stony) beach, the hillfort, and an air museum so is also worth a visit.
if you’re intending doing the castles, I’d suggest a Cadw membership as you’ll make it back even if you only visit Beau / Harlech / Caernarfon and it gives you access to loads of other places, including the Tudor House in Conwy.
hope this helps.