i'm happy with both my GTO and my '04 cobra, and would likely be thrilled with a vette as well. whatever you do, i would stay away from an older car--
and obviously, the more you do to a car, the less fun it will be as a daily driver. my GTO has an intake and catback and is amazingly comfortable to drive ANY distance, or any amount of time. when i bought it, i immediately drove it 500 miles from wisconsin to ohio, and felt like i was on a leather sofa the whole time--and like mentioned, at 70mph, 25mpg is achievable.
i would say pick amongst these cars and you won't go wrong: '03/04 cobra, c5zo6, ls2GTO, G8, newer sti or evo, and a low mileage, near stock fbody (if one still exists). these are all easily had for less than $30k (except the G8, and maybe the vette). stay away from anything pre-2000, avoid beat-on fbodies, and fox body stangs, as well as '05+ stangs.
a G8 would be great if you could find one under $30k---great power, full factory warranty, and a NEW car.