Pause and Exit buttons. Close enough to be handy, but far enough away to not accidentally get hit.
I've sent this link before but he has a nicely done Frankenpanel (I know, those get some bad press around these parts

) but his page has a nice explanation of why he laid out the controls that he did.
Personally, I put the coin and player start buttons on the vertical panels between the top and bottom level. That seemed to work better for me and it helped to declutter the bottom level.