Robert,
I live in Pontypool and you have just missed our last MVT meeting, which are held on the third Thursday of the month.
Near to Cardiff you could try ....
Castell Coch
Brecon Military Museum
Caerphilly Motoring Festival at Llanciaich Fawr - 5th July
St Fagans
Big Pit Mining Museum at Blaenavon
Blaenavon Heritage Railway 1940's weekend - 11th & 12 July
Abergavenny [lovely little market town] and the mountain road over to Hay on Wye
Loads of Castles - Abergavenny, Whitecastle, Grosmont, Skenfrith, Raglan. And they're all within 12 miles of each other, loads further afield...
Usk Tank School
Cosmeston Lakes Medieval Village
Google the above for more details
If you want any further I'll pm my tel. no.