I went to the Indycar race there twice. The first time I got a bleacher seat overlooking Hurry Downs, which is between turns 7 & 8. Turn 8 is a good passing spot. I liked this spot because a concession stand was close to the bleachers. (If you're at Road America, you have to buy bratwursts.)
The second time, I got a spot along the fence going into turn 5, which is the prime passing spot for the whole course. But standing for the whole race was a pain. There are also bleacher seats on a hill outside of turn 6 overlooking turn 5. This is a good vantage point because you can see passes developing going into turn 5, as well as watch the cars accelerating up the hill from Canada Corner.
The other major passing spot is Canada Corner, which is turn 12. There was no easy way to get there the last time I spectated at Road America about 13 years ago. If you want to see all the corners at RA, you better have comfortable walking shoes.