Can't believe that you've never been to Wales as it sounds like the perfect destination for you. Physical activity for the kids, lovely scenery for DH to admire, rivers, wildlife aplenty (don't know about the hot tubs tho'!).
It's also a lot cheaper than posher places like Devon & Cornwall, esp for self catering cottages.
Pembrokeshire in S Wales is lovely - stunning coast and beaches (Tenby is a pretty, buzzy town, lots to see and would make a great base). Mid Wales, around Rhyader and towards the coast has lots of rivers, green hills and amazing red kites (see them being fed at Nant Yr Arian near Aberystwyth - it's astonishing) but it?s quite remote.
Really, I?d be tempted to head North if I were you ? we love Llanberis, a village at the foot of Snowdon and a mecca for walkers, climbers and outdoor enthusiasts. There?s all sorts of physical outdoorsy stuff for active teenagers to be getting on with (including climbing the highest mountain in the country), bouldering, scuba-diving in a slate quarry at Padarn Country Park, a huge lake for boating, canoeing & fishing, a hydroelectric power station which offers tours and the Welsh Slate Museum where you could easily fill a wet day ? the place is big enough that you could spend a few days there without having to use the car and yet small enough that you can give older kids the freedom to wander off on their own. A great ice-cream parlour too and a legendary climbers ?caff? called Pete?s Eats. There is plenty to see within half an hour?s drive too ? Anglesey, Portmeirion, beautiful mountain ranges, castles, coast etc. Well connected as far as buses are concerned too.
We love Wales ? what?s not to like?!