Cornwall is fantastic, there's plenty to do....
- Walk along the north coast between St. Ives and Lands End. Beautiful coast, old tin derelict tin mines to see.
- More beaches than you could shake a stick at. Being a national trust member is good, a lot of the beach car parks are owned by the NT, so you get free parking.
- St Ives has art galleries galore, and a weekly jazz club on Tuesday nights.
- Cornwall has a fantastic music scene. Check some gigs out. A lot of pubs have bands on every friday and saturday night. Particular recommendation of mine is the Driftwood Spars Hotel in St Agnes. Queen played there when they were all Cornish teenagers in a band together.
- Go out in Truro or Falmouth. Newquay is horrendous, but the aforementioned towns have some nice bars, and a good friendly nightlife atmosphere.
- Eden project, as you say, is a must.
- Buy a pastie from WC Rowes in Truro. Best pasties I ever ate.
I'm sure there's plenty more, I spent my uni years down there, and don't recall being bored for one second!