Well, my husband once used a stapler to staple up the hem of a pair of pants, but I wouldn't recommend that.
If you didn't want to use a needle and thread like Liz suggests, then I'd go to your nearest fabric or craft store and ask for the fusible web tape that Carter has linked to in her reply. It's super simple to use because you just fold up the hem to wherever you want (and I wouldn't even bother removing the old hem unless you have enough turnover to cut it off), sandwich the tape between the two pieces of fabric positioning just beneath the top edge away from the fold, and then use a hot iron with a press cloth so the tape melts and fuses the fabric together. If you don't want even bother with an iron, you could also use dots of fabric glue to hold the turned under fabric in place--just be careful if the fabric is very lightweight because the spots of glue can show through on some fabrics.
Actually, my quick and dirty method for hemming a pair of casual pants that only need to be shortened an inch or two is to just fold under the original hem and then quickly stitch it in place either with my sewing machine or by hand. with a needle and thread. It makes the hem a bit thicker but I've found that actually helps some pants drape a bit better.
ETA: It's true that the fusible web won't last through repeated laundering, but you'll have more than enough in the roll to do a quick fix if the hem starts to fall down--and maternity pants usually aren't worn forever!
