We always stay near Penzance & really like that area, you can get to either coast easily. We go in August most years and never find it too busy, sure car parks at popular beaches, but there's plenty of places you can get to off the beaten track. Loads of good walking. Cheap campsites.
Places we've enjoyed visiting are St Michael's Mount (even that wasn't too busy the day we went, plenty of space there to wander), Minnack Theatre (saw Levellers have a sold out gig coming up there soon! we saw a kid's play, well worth it), neolithic stuff (Chysauster Bronze Age village, small but interesting, want to go to more of these places), and if you like mines/industrial stuff then
Geevor Tin Mine: Home is worth a visit. And there's loads of abandoned ones dotted about to just stop and look at. Oh the Telegraph Museum at Porthcurno is also good, don't know if you have kids but it's very kid friendly too, and you get to go underground as well to the war rooms. So much more I want to explore when I'm next there! Penzance has lots, art gallery, harbour, botanic gardens, love Penzance, it has such a nice down to earth feel, not wall to wall touristy.
Geevor
Favourite beaches Porthcurno (small) and Godrevy (stretches on for miles)
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