You're probably familiar with
Ranma 1/2, but I'd suggest reading it if you haven't. It's entertaining all the way through, and 38 volumes should last you a bit.
More action comedy to be had with
Angel Densetsu, one of my very favorite manga. Clocks in at 15 volumes.
Yokohama Kaidashi Kikou - 14 volumes
Feels like literature in a sea of manga "genre fiction." Great slice-of-life science fiction.
Gokusen - 15 volumes
Hilarious, a josei take on delinquent manga.
Kanata Kara - 14 volumes
The best fantasy epic to be found in manga, and it's shoujo! *gasp*
Please Save My Earth - 12 volumes
I guess I'll throw some more shoujo out there while I'm at it. This definitely falls into the "sci-fi/fantasy/psychological" category... all three, actually.