So you forgot her birthday again? ;-)
Last minute gifts are always a bit of a headache to get organised. You will need something that you can arrange at the very last minute, buy online and print out a gift certificate, or buy online and print out the booking information and perhaps some web pages.
Here's some ideas:
Chocolate Now have a nice feature where you purchase (and print out) a gift voucher. With this chocolates voucher the fortunate person being bought for can then access the website and design their own box of chocolates, even down to choosing their favourite flavours.
It may already be past the time for this but some merchants such as Interflora offer a same day delivery facility if your order is put in early enough.
How about taking her out for dinner. At minimum this is something you can arrange by phone (and, if required, make out that you booked it weeks ago!).
If you do not have time to wait for delivery then you could buy her a Amazon Gift Certificates gift voucher which you can print out from their site immediately. With this she can buy CDs, DVDs, Books, Computer Games and an entire host of other things.
If you can get to the shops then, as well as mail order, Boots offers a reserve and collect service where you can check out local stocked items and reserve products for collection. Ideal for that last minute bottle of scent or aftershave!
Would she like a surprise weekend away? Check out Warner Leisure Hotels and see if they have any last minute discounts. You can print out the booking information and a couple of pictures of the hotel from their website.
How about chocolate delivered each month for a year. Get her a paid-up account with the Hotel Chocolat tasting club. You can print out their subscription certificate and dash down the high street for a box of off-the-shelf chocolates to keep her going for now.
Good luck!