Money, especially in multiples of 18, is very common and will be just fine. Gift cards also.
Girls do get a lot of jewelry and purses. Boys and girls both get Judaica (candlesticks, kiddush cups, chanuka menorahs, charity boxes, etc.), Israel Bonds, US bonds, and lots of books of all kinds.
Also popular: jewelry box or tree, jewelry travel cases, photo albums, wallet, watch, binoculars, toilet kits. Things for camp.
One very very clever gift my son, even then a baseball nut (today he is a sportscaster) received was a few shares of stock in a baseball team that was at the time publicly traded. He got a lot of other baseball stuff, too. My daughter loved a gift of CDs from four new Broadway shows. You need to know the kid to hit it out of the park that well, though.
My favorite bar mitzvah gift to give is an electric razor. Something a man needs but a boy doesn't. The boys LOVE it, of course! Especially the ones who won't need it for years. I haven't found a perfect analog for girls (when I was a kid, a hairdryer served the purpose -- and I don't mean a blow dryer! -- I still remember how thrilled I was to get that, and I even remember the card: "for the new teenager - you'll need this for those last minute social engagements" -- it made me feel so grown up.). I often give girls two books,: Our Bodies, Our Selves, a women's health book that every woman needs, and a book on personal finances aimed at teenage girls. Same idea, but needless to say, the girls don't get a thrill out of it like the boys do with the razors.