We have a very large family and it seems that there's always someone pregnant - in 08 we had a baby born in Aug, Sept, Oct, Nov, and Dec. So I've learned what things are the biggest hits at showers.
Handmade Blankets - pick out fabrics that have the parents inteste in them (one Dad likes pool alot, I found great flannel fabric with pool balls on it, loved it) try to make it unique
If you know how to knit or crochet - booties and tiny socks are wonderful - especially with babies born in the colder climates or seasons
burp cloths - using a prefolded cloth diaper you can fancy it up by adding a strip of fabric
bibs - making these to match the blankets or burb cloths are always cute
Sleep saks are wonderful for babies - just remember to make them in a fabric for the season
Baby Legs - or little leg warmers are great too, perfect for layering or under a little dress (you don't have to wrestle with getting something over the diaper which is a plus when changing tons of diapers a day
Tiny headbands in soft fabrics
a carseat cover - again use fabric appropriate to the season
Tiny clothes are well recieved, especially when you make them - beware though you may get request for many more in the future
Mittens - newborn size and slightly larger - these things are always disappearing
If they are going to use a pacifier you can make a holder thingy (don't know what they're called, but it's basically a small strip of fabric or ribbon sewn to the end of a clip to attach to baby so it won't fall to the floor and get dirty)
A breastfeeding Momma might like a "hooter hider", a large blanket type thing that allows her to feed without an audience
Handmade wash clothes and towels are great too, and they "grow" with the baby
I usually also get diapers in either cloth or disposable depending on Mom's preference, these make super cute wreaths. I bought a wreath form at the Dollar Store, wrapped the diapers around securing with a small hair band and jazzed it up with ribbon and tiny baby items like nail clippers and such. A mega hit at all three showers, and it doubles as a decoration for the shower, always a plus.
Good Luck! And you have it right, babies tend to come in "crops" as my Grandma calls them.