Like a lot of other people I'd recommend Geography or maybe ICT, I don't study ICT but a girl in my year picked up the entire A Level in year 13 due to illness at AS and still managed to get an A.
Also, even though General Studies isn't accepted by all universities, I'd recommend taking it if you're just looking for a bit of a grade boost. Nobody in our year did any revision as we weren't assigned any lessons for the subject, but were still obliged to take it- our teachers just advised us to read the papers and watch the news in the run up to the exam. Considering you'd have to put in very little time prior to the exam it might be worth doing just to show you have some critical thinking and essay writing skills. Plus, a friend last year missed an offer from Edinburgh to do Languages by one grade, but they accepted her anyway and stated that it was partially to do with her A in General Studies (despite the fact her original offer didn't include the subject). Obviously it's not going to be life changing in its importance but it takes up so little time you might as well consider it.