Fun? Just make a hollow cube textured with GENERIC_CRETE and gfill it with 50 monster_hgrunt and monster_aslave's with no info_nodes and let the "fun" begin!
But seriously, my advice is look at Half-Life, play some of that game again, and look at why their levels are fun, and the amount of detail they put in.
[edit] I just snezzed and farted at the same time, the fart was more a sort of 'wet wheeze' - I feel sticky =/