There's a campsite behind the TT Grandstand, Nobles Park, but I'm not sure it's open during TT week. I've also stopped at the site at Cronkdhoo, near Greba Castle on the TT course which was fine (nice pub down the road too). There's a TT campsite near Braddan Bridge, just outside of Douglas.
If you're not into drinking all day just drive around, there's some amazing scenery and some cracking roads - the B10 from Brandywell to Barregarrow and the A36 from Port Erin to Foxdale are two of my favourites