If you choose to buy a toy that makes noise, try to pick one with an on/off switch. Adjustable volume is also a plus.
Make it age appropriate, please - if the child is turning one, you should not get a toy for 3 years+. No matter how smart the birthday boy/girl is, this toy is too old and may contain small parts that the wee one could choke on.
If you're buying clothes, please include a gift receipt in case the item is too large/small. The sizes on children's clothing are tricky and all children are built differently.
Books make good gifts - I've never heard a parent complain that their kid has too many books. However, when making your selection, stay away from the classics. Chances are good s/he already has a copy (or three) of Good Night Moon, Green Eggs and Ham, and the Hungry Little Caterpillar.
If you don't know what to get, get a gift card or a savings bond. Parents love them.
Toddlers love bubbles. LOVE them. LOVE. Any gift that involves bubbles is sure to be a hit.