I went to Spa in 2007, and General Admission is quite a good option.
If I were you I'd go for the full three days so you can move around the whole circuit and experience all the corners.
So, where to watch...
The good thing about Spa is that the circuit is so long they don't have nearly enough crowd control so if you slip into the woods on the infield you can reappear almost anywhere.
Allow me to inflict some of my crappy photograpy on you...
I took the following shot just before Stavelot. This is where they park the F1 trucks and surplus traliers on the infield. We stood between a few for ages and had an awesome view. No one even batted an eyelid. Very close to the action...

Eau Rouge is clearly the most awesome corner at the circuit but is tricky to view without a grandstand ticket.
BUT... head for the F1 merchanising area which is right at the bottom of Eau Rouge.
If you get there earlier enough on Saturday you can get an awesome spot looking straight up at Eau Rouge for qualifying. We got there 90 minutes before the session started. Filled up quick, but we had a good spot. Presume the same is true on raceday. We were right beside the fence as the cars whipped past inches away, you saw awesome clouds of yellowish plank dust as they went under full compression...

For raceday we headed up to the long uphill straight which ends at Les Combes.
There is a great grass bank all the way along this straight.
You could position yourself at the exit of Kemmel, where you see the cars pop up out of Eau Rouge which is quite good (just prior to the Allianz bridge you see in the shot below).
If you head about two thirds of the way up the straight you'll be at the point where lots of cars are pulling out of the slip stream to try and overtake.
There is also a big screen TV here and you can see across the valley all the way over to the Bus Stop.
A tip: Get here early so you can get a good spot on the rocky hillside. Take something to put on the ground, like a piece of plastic or something. Lots of holes have been dug out over the years so you can make yourself quite comfortable. After that, get your elbows out. When we were there some of the Belgians were pretty pushy and rude, trying to get onto our patch...

You can get up to Les Combes in the GA area, but you'd have to be there mega early and the view isn't that great.
You can also go over to the straight between Malmedy and the Rivage hairpin.
We went there for the first session on the Friday in 2007, just in time to watch Felipe Massa lock up and end up in the gravel (below). Trouble with this corner is that there is no banking so everyone is pressed against the wire. So you'd have to be there mega early or not bother. I think there are better options.

Another good spot is the entry to Pouhon, there is a steep grass bank up the hill and a super fast corner. You are quite far from the action though.

The Fagnes corners are OK, but you don't have the benefit of being up a bank like the other corners I've mentioned so the view isn't quite as good. Would probably get quite crowded on race day.
I'd recommend the sitting between the trees about halfway between Blanchimont and the Bus Stop at the end of the lap.
You'll have to get there early to nab a spot, but you've can get quite a nice perspective of the cars flying around then hitting the brakes for Bus Stop.
Obviously the closer to Bus Stop you go, the bigger the crowds.

We couldn't find a way to get anywhere near La Source, but managed to see most of the rest of the lap.
Hope this help, Spa is totally awesome. Take a pair of comfy shoes because you will be doing
a lot of walking if you want to see some of the corners I've mentioned. I didn't rain in 2007 but I've heard it does, so a raincoat would be handy.
Worth it thought.