Last time we were in Disney I didn't make Sci Fi reservations because I thought it was time to try something else. Every time we walked by the place the kids gave me heck about it - so we ended up stopping by at an off time, I think around 2:30 or so, and asking if we could go in for milkshakes.
They had plenty of room then and they were happy to seat us. I think the waitress was happy just to have a paying table, even if we didn't order and tip for a full meal. I did tip about 30% though 'cause I felt kind of guilty and she was nice.