I was fortunate to get to one of the best races ever....Catalunya last year all 3 days of it!
Our stand was on the second last corner. The circuit had an outer fence (which allowed you to walk freely round the entire circuit) then an inner fence (which was to allow you access to a particular stand) but if you went to your seat and wante out, you were given a pass which allowed you back in. So if you wanted to walk all the way round, it wasnt a problem.
Obviously take your camera and I found a small set of binoculars was good to have.
Where we were seated, we could see the last 4-5 corners too, allbeit they were a bit away. We also froze a big bottle of watter the night before and took that along. They were charging 5 for a 300ml bottle of Fanta! We also made up some sandwiches as it is a very long day.
You tend to find only the official merchants sell clothing inside the circuit with a huge amount of stands outside.