Just a blank panel where the radio would go too. Just the road noise to keep you company...
Most people go with 6.5" speakers in the front. As said depth is a real concern as the window intrudes into the space. The kit provides a spacer to keep the glass and the speaker magnet from occupying the same space at the same time. (Don't get greedy though.) If you go with components and need a tweeter speaker brackets, that kit in the link has them. The factory upgrade would have them already. You would still have to fab a simple bracket for the crossover, but there is a nifty space inside the arm rest.
In the rear some claim you can run 6.5", 5.25" definitely fit and 4" is stock.
I went with components up front and 4" in the rear for fill. I can't recommend anything in particular because I did my install back in 2003.