Best accessory for a starting photographer for under $100?

My sister plans on becoming a photographer and going to school next year for it. She already has two cameras-- Nikon D3000 and Nikon D7000 that she bought recently. She has the standard lens and a couple other lens that came with the kit she bought and a carrying case. Things I know she needs, but that I can't buy her because they are too expensive: flash, other lens, etc. Any suggestion on what I can get her (its for Christmas) that could help her that would be under $100? Even if its from Ebay, where I could get a better deal. I know she'll need a better photo editing software program, but only has paid subscription "Picnik" for now. Any suggestions? Thanks.

Added (1). Additional info: She has mainly been doing outdoor photography with families, senior photos, and just recently did a couple weddings.

Added (2). Thanks for the ideas! She does have a 16 GB memory card. I would rather give her a gift, but if nothing else I could do the gift card.

Things always appreciated are extra batteries (only get Nikon… D3000 http://www.bhphotovideo.com/...ttery.html and D7000 http://www.bhphotovideo.com/...00mAh.html ) and memory cards (4GB class 10)

I prefer getting gift cards to an actual local camera shop (not bestbuy/walmart) or B&H Photo… If I don't buy things from the camera shop, it is always B&H… Http://www.bhphotovideo.com/find/giftCard.jsp

I gotta go with B. E.I. There - gift cards are the better choice. You don't necessarily know what she wants/needs, and don't necessarily know whether you're getting a deal on it, especially on ebay or craigslist. Give her the card and let her pick out what she needs/wants.

Since she'll be taking photos outdoors she might be needing a polarizing filter which cost under $100. A neutral density filter set will cost less than a $100. UV and Skylight filters cost less than a $100.

You can combine a linear and circular polarizing filter to create a variable neutral density filter but it might go beyond your budget.

Adobe Photoshop Elements might be available for around $100.It is the little brother of Photoshop, and will do most of what she might need. Certainly better than Picnik.

I also second the gift card from B&H! I would love to get one!

An extra flash memory card or two can be pretty useful, but you never know if she already has all she needs of those or whatever.

I agree that a gift card or gift certificate would be most useful. With that, your sister can apply it to purchase EXACTLY what she needs. For all you know, it may turn out that she needs a particular textbook more than she needs any accessory.

Buy her Umbrella lights. I got 2 for $50. They are really helpful for photoshoots