Are you going all 3 days? Spend Friday walking around the whole track. There are lots of great spots, its down to personal preference.
To give you an idea, going in the direction of the track, from the final chicane, down and around La Source, and all the way back up to the top of Eau Rouge is grandstand only, with no GA. You can peer over some fences and see things, or look up from the merchandising stands (top right of the photo above). But from the top of Eau Rouge around the whole rest of the track there are just massive grass banks. There are a couple of grandstands, but its basically all GA.
All down Kemmel straight is a big bank, with a big screen on the other side. Les Combes is a great overtaking spot, but unfortunately due to an access road (that used to be the old track) you cannot spectate right on the corner. There is then a tunnel under Malmedy that means you go from the outside of the track to the inside, and can get right on the inside of Rivage, which is a fantastic spot for close up photos as the cars are not going to fast. Then on down to Pouhon is a MASSIVE drop. At first the spectator banking stays up high so you get great birds eye views, and you can just make out the pits in the distance. The bank then drops down around Pouhon which is a good spot (sat here 2 years now) with a big screen across the track. Just a warning, walking back up that hill is a killer! There is then a large flat area at track level around the inside of Les Fanges. Not idea for spectating, but great for photos and getting a sense of speed. Then there is a tunnel under Stavelot that goes out to the red Zone parking, and to a grandstand thats on the outside of the corner, or you can continue inside the track, lower than it down by the river, and under a tunnel half way between Stavelot and Blanchimont. You are now back on the outside of the track. There is a path with some trees between you and the cars that runs all the way to the Bus Stop. Epic sensation of speed and noise through here, and some good fast spots when the trees break. Then when you get to the Bus Stop there is a steep hill on the outside with a massive rock that can offer a good view. Then, as GA, you will have to go under a tunnel under the chicane and around the back of the pits. This goes accross and then you go back under Eau Rouge. You can view from a small area on the inside of Eau Rouge. Then back under the tunnel and you are back on the outside of the track at Eau Rouge, in all the merchandising stands. The footpath then goes up behind the grandstands on the outside of Eau Rouge and you pop out at the top of Eau Rouge, back at the start of the Kemmel straight.
And that concludes our guided tour of Spa
. There is also a footpath through the woods from Pouhon straight up to the inside of Eau Rouge as a more direct way to get from one end of the track to the other.
As I said though, walk around the whole track on Friday to decide where you want to go Sat and Sun. Oh, and be prepared for hills! The track has some epic elevation changes that the TV just doesn't show. The whole of Kemmel is up hill, and the drop from Les Combes down to Pouhon is probubly 30 meters! Just look at the picture above, the buildings on the top left are the back of the pit garages on the main straight!