If you want to follow the race action, I'd recommend the grandstand in front of the old pit boxes; There is a big screen where you can follow the race and you can see Eau Rouge, the drivers did some nice moves there. For the start, the grandstand on the main straight of course, there should also be a screen. But you can also follow the track around, there are footways all around the track. And take some time to go to the paddock. Especially the teams from the lower categories (everything except the WEC teams) often don't have a tight schedule and they are more often than not willing to talk to visitors about their cars and the races. At least that's what I was experiencing last year and this year as a Team Manager of a GT4 team as we are always the lower category.
What you absolutely must do is go to the bottom of Eau Rouge, you can get very close to the track from the grandstands there; Then you can see how steep it really is. It looks steep in TV, but that is nothing in comparison to real life. It's even more impressive when you are standing on top of it, but I fear that might not be possible that weekend.
