Hi Joe,
We were across to the mainland recently and went the Holyhead route.
Beaumaris is a nice spot and it's a nice drive to it along the shore of the Menai Strait. The campsite is a bit far out of the town for my liking.
For a first-night stop-over we used a site called Treborth which is about a mile from the crossing over from Anglesey to the mainland. It's a basic site, but well placed if you want to come out for a short stroll to view the bridges, or dander back across the bridge into Menai Bridge town. There's a pub with a nice view over the bridge on the mainland side, just a few hundred yds from the site
Llandudno and Conwy on the North coast are nice places to visit, or you can take the roads up into Snowdonia, bits of it are very alpine.
Electric Mountain at Llanberis is real Big Boys' Toys stuff - hydro electric power station carved into a mountain. The tour is about 40 minutes, though the dog will have the stay in the van!
Llanberis is where you get the train ride up to Snowdow IIRC.
Betys-y-coed is a nice spot in the forest, lots of visitors, nice for a dander around, coffee stop and loads of shops for clothing for hikers, cyclists, etc.
I can't recall much else of a visit through Mid-Wales a few years ago, but when you're heading more towards Bath, Tewkesbury has a nice campsite right in the town. It's the Club site they always show on the news when Tewkesbury floods. You're looking across to the Abbey from the site, and there are walks to the rivers.
Bath is nice. We stayed at a beautiful campsite about 4 miles away and walked in along the tow-path. Can't recall the name of it right now.