Having been to Spa in 2013, 2015, 2016 & 2017 I can confirm that the grandstands are open and that you don’t need a reserved seat. If you want the best seat then get there early!
As previously suggested the best solution is to get your walking shoes on and get around the track. The climb up from Eau Rouge to Raidillon is steep, but the Kemmel Straight is easier. Get up to Bruxelles and what the cars go around 180 degrees around you. Walk down Pouhon, there are chips and beers at the bottom!