As someone who is both in my 20's and has worked as a full time nanny for years both birthday gifts and holiday gifts are a great way to know that you are appreciated. Depending upon how many hours she works for you should dictate the level of intimacy and cost of your gift. For example, I have gotten books and gift cards for small amounts (under $100) from families I worked less than 20 hours a week for. For a full time position a gift of more substance is more appropriate. For example for my birthday this year the family I work for gave me a Cooks Illustrated cookbook and a $200 gift certificate to Williams-Sonoma.
If you are buying a gift certificate (speaking from experience) stay away from Starbucks (unless she shows up every day with a cup in hand), Costco (being 22 it is doubtful she's a member), or any other food store. Clothing stores like Urban Outfitters, Anthropologie, Express or JCrew are much more a 22 year-olds speed (unless she's preppy then Gap or Banana Republic). If she likes to cook William-Sonoma, Crate & Barrel, or Sur La Table.
But the BEST gift I ever got was a gift certificate to a local spa that I could use for a hot tub, massage or facial.
Hope that helps!