Whilst washing up made me realise that now the bridge is free and you’ve found your passports there is lots of stuff in Wales that we’ve done or is on the do list for when we visit the family there.
Thinking of things aside from outdoors stuff which is great but less so if very wet.
Lots of castles: Chepstow, Raglan, Caerphilly, Castle Coch. An annual family ticket may offer best value if doing 3 or more. https://cadw.gov.wales/
Go Roman for the day in Caerleon with the bath house, amphitheatre, barracks and museum.
The national museum in Cardiff is good and easy to get to on the train from Bristol.
Best of all is St. Fagans museum of history just west of Cardiff. Indoor and outdoor bits of museum and bits of ‘living’ museum type bits. An Elizabethan manor thrown in for good measure. Easily a whole weekend of stuff to do with kids.
https://museum.wales/stfagans/
Seem to remember that Bristol is worth visiting too 😉