Since it is a milestone birthday for a sweet niece, I would definitely to in the direction of "there are some things money can't buy" (remember the mastercard commercial with that theme?)...maybe include a mastercard gift card in some amount to continue the theme, but then focus on some sort of non commercial keepsake that she will treasure forever, not be lost in the rest of the materialism...like love, hope, dreams, creativity, risk taking, gaining wisdom (not just knowledge) - all things no amount of money can buy. I noticed someone suggested a scrapbook...good thought! Wouldn't it be fun to write a "journal entry" of what it was like to be 16 for you, ending with what your hopes are for her in the coming year as well as the future? Maybe get all the women in the family to do the same...and guys too! If you kept a diary at that age, maybe share some of your own musing in her book. And have pages for "life is like..." with current events, prices of bread, eggs, and milk and gas...
And to keep with that old MasterCard commercial theme...have some pages already made up for material things she has already gotten, with an estimated dollar amount, to acknowledge those things to, but then leading into what cannot be purchased!
Have fun!