Twinkly lights, floral garland, little pumpkins or trinkets to put on her desk/shelf, candy/cookies (bonus if you include extra for her room/suite/housemate(s)), a pillow/case or throw/blanket for her bed, a festive T-shirt or tote, money for her to splurge on a spooky film, pizza place, or local décor store...
Honestly, she'll probably be too grateful that she received a package at all or too distracted by school to be picky about the specifics. And if she's anything like the majority of college kids I've known (and recently was), childish isn't necessarily a bad thing. It's nice to be babied once in a while, especially on holidays when even the toughest might feel a little family/homesick. Even those who seem intellectually mature may have a great deal of emotional maturity to go. And who doesn't savor or yearn for their days of youth once in a while? Major brownie points for you, auntie. ;-)