have you thought about asking what your partner would like? maybe put her name and 2010-2011 in cutouts on the front?

I am going to cut out some manga pages or maybe just draw something on the cover. If you partner likes cute stuff you can always use some memos or stationery.

My favorite way of decorating comp. books (or books in general) is to take a sketchbook that's larger than the book you're using, tear out some pages, and glue the pages to the outside/inside covers (you may have to use a couple pages, just to cover up the original design). ^.^ I like to take out some pens and doodle or write silly things on the cover.
Another thing you could do, that's a lot slower but also fun, is just buy some stickers from the store over a few months and gradually cover up the design. Or, like you said, you could collage with magazine pictures.
Or all three options, together. :)
Just have fun and if you decide you don't like it, you can always cover it with something else afterward.