So as others have said, you
can "collect" FPs. You can't get another one until your window opens, but you CAN hold onto the first one (not using it until later) while getting another one.
Last Friday, for example:
8:05 am got FP for Space Mtn (return time 8:40)
8:45 am got FP for Thunder Mtn (return time 9:40)
10:00 am got FP for Splash Mtn (return time 10:45)
10:45 tried to get FP for Indiana Jones, but ride was down
So by this point we have "collected" 3 (could have been 4) sets of FPs without using anything. Meantime, we did several rides in Fantasyland, part of Toontown, rode the train, Haunted Mansion, etc. -- the non-FP stuff. We just got a FP for whatever we were close to when the next return time opened.
We didn't use the Splash and Thunder FPs until afternoon and the Space FPs until almost dinnertime.
We don't absolutely maximize the possibilities like some DISers can, but we still skipped some sizeable lines that afternoon. So that's a real-life example with only a little planning and some clock-watching.
Here's me singing in the Thunder Mtn FP return line:

"I'm skipping, skipping, skipping, the line"

: ... I threatened DD11 that I would actually skip too ... This is DD11:
PHXscuba