Good thing about the Indy circuit is you can see the whole thing. If the GP track is in use you don't get to see half the race.
I'd like to go for BTCC or GTs. GTs are the better option as the circuit is far less crowded. Saturday practice or quali is a good one. Usually only £5 - £10 entry and you get the circuit to yourself.
Best of all though is Lydden for either the oval racers or rally x. Again, cheap to get in, you can see the whole circuit but the difference is you can see almost the whole circuit without getting out of the car. You can sit there and have a smoke or whatever. Its great.