Last time we went to Spa there wasn't any reserved seating, there is an open grandstand in the run down to Eau Rouge, plus of course you can walk up around the outside, plenty of viewing available all around the circuit. There are also grandstands at the start line, and on the exit of La Source.
Just take your time and potter aound, oh and take about 10 variations of clothing, you can see four seasons in one race, let alone day. Mind you we've been four times, and it has not rained a drop