Diapers are always needed with a new baby. It's the most practical and useful gift that you can give a family with a new baby.
As the host for the baby shower, you can purchase a package of newborn diapers, and add a sticky note to each one (double-sided tape is also useful for this). As each guest arrives, give them a diaper to write a note of encouragement to the mommy and daddy to be. The first few weeks, even months of parenthood can be tough, so encouragement can help reduce some stress and create smiles during diaper duty.
It's also a great temporary keepsake for the new parents and baby.