Definitely agree w/ using their registry as a guide. You can still buy items off the registry and make it unique. I'm a big advocate of gift receipts, even if you're getting items off the registry. B/c as I recently learned, Target for example, will only let you return items with a receipt. So if you get a duplicate, even if it's off of your registry, they won't even give you store credit without a receipt!
A big hit that I've done using registry items (or not) for baby showers is "pea in the pod." You'll need- a green blanket, 6 pairs of white or green bootie socks, ribbon, and a soft stuffed baby doll (girl for a girl, if it's a boy i try to find like a boy doll in a sports uniform of some sort or even a puppy). Lay the green (try to find a shade that matches w/ their colors) blanket flat, then roll 1/3 of each side (long ways) to the middle. So it should be long w/ 2 rolls. Fasten each end (leaving a couple inches) by tying a ribbon in a bow. Then, stuff the middle with the doll, and separate the bootie socks so they're individual. Roll the bootie socks to make the "peas" around the baby, and hence your "pea in the pod." I usually attach a cardboard square on one of the ribbon ends that says "a pea in the pod for your pea in the pod." It gets lots of ooohs and aaahs, and I've had a lot of people tell me they'll be copying my idea for other baby showers.
The towel cake/diaper cakes are also cute. For bridal, you can use kitchen/bath towels they registered for, or for the baby shower use either diapers, bath towels, receiving blankets, etc., off of their registry. Wrap each "cake layer" w/ a thick ribbon matching their color scheme and you can insert little items they registered for around the layers or as the top of the layer. for baby ie- toys, rattles, socks, bottles, pacifiers, etc. For bridal- kitchen utensils (spatula, spoon, ladel, etc) are easy to stuff in the towels.