I'm a COTA regular (many bike races, a few car races, several trackdays), and IMO, T6 is the best general admission spot, and T15 is the best grandstand seating. I did GA only for many years, but have upgraded to T15 seats. I watch practices from T3 to T9, the races, qualifying, etc from the seats at 15. T15 makes it VERY easy and low effort. Food, bathrooms and entry are all close by. Getting to the in-field vendor area is fairly quick, and the entrance is also close-ish. But, its pricey these days, and the view from the GA area is really quite good. There's some food back there, too, and bathrooms - but it's a hike to get there. The big screen viewing is better from T6 than from T15 (for the last couple years at least - they seem to move around a little). During MotoGP, you can't access the fun stuff like Charliez mentions: paddock or main grandstands without special passes.
You can also volunteer as a track marshal. I did T14 and just loved it. The view from right on the track is unbeatable. We had rain that year, so stayed busy helping bikes off the track. That makes for VERY long days, but it's free access - and you even (used to?) get discounts on RideSmart trackdays...