Instow is your best bet for flat water as it's sheltered from swell. Easy to launch from too.
Westward Ho!/Northam would perhaps be a better choice as there's less rocks, although I'm not much of a windsurfer and don't really know much about it. There's normally plenty of kitesurfers there and at Saunton when it's a strong onshore. There is a local windsurfing club that could offer better advice
The riding around Barnstaple isn't epic (hence why I bought a road bike), but it may offer some fun as you're about this way.