So, you have a map? Let's make stuff in there! (There are many of these in basic sets of rules in different games, but this is for fun).
1. It's empty. Or is it? Roll again. If the result is "1" again, it really is empty! If the result is something else, the room looks like empty, but there's [the result] hidden from sight.
2-5. Some wandering monsters. Roll d6: "1" those are weaker than normal (-1d4 HD). "2-5" they are as written. "6" they are stronger (+1d4) HD.
6-7. Trap or traps! 15 (1-3 on d20)% (one of the) trap(s) is save versus death or die. There is 1 trap 75% or 1d4 traps 25%.
8. Treasure! Roll treasure table or decide. There's 10% change that the treasure or part of it is magical (25% change it is bad) item, scrolls, whatnot.
9. Special! This room is somehow special. How it is special is up to you. Kitchen, guard barracks, treasury room are not special. Teleport to Mars, altered gravity, weird science are special. In special room roll again to see what else is in there. If the roll result is "9" special again, the room is DOUBLE-SPECIAL (or infinitely frikkin' special if you roll 10 and keep rolling special rooms). DOUBLE+ SPECIAL room might be a WEIRD SCIENCE ROOM IN MARS (no explanation needed, it's that special).
10. Roll twice and combine (if you roll lots of "10"s you get a room with lots of stuff in).