My recommendation would depend on whether you consider your interest in biology to be a side-interest or a serious enough interest to consider changing majors. If it's a side-interest, you might want to enroll in a non-majors biology course...that will be light enough to not overwhelm you in conjunction with your physics courses, but give you a nice introduction to the subject. If you're seriously considering it as a major or future career focus, then it's best to take the introductory or general biology course intended for biology majors sooner than later so you'll have time to take the later coursework which will all require that as a prerequisite. Unless you're a complete masochist, I wouldn't recommend attempting a double major because there isn't enough overlap, and both are difficult and time-consuming majors. Generally, the difference between a biology major and minor is the amount of required chemistry courses, so even that route doesn't help a lot (though, check with your university's specific requirements to confirm that if it is something you might consider).