Critical Thinking really won't take up too much of your time. I do it at College, and I only have 1 lesson (1hr 20mins) of it per week, and no homework. There is really no actual content to learn - there are various techniques of how to identify conclusions, assumptions, flaws etc, but most of it is common sense.
Its not exactly the most exciting subject, but universities do like to see it as an extra as it can help to show natural ability, rather then how much you can cram revision.
Give it a go, you've nothing to lose by trying