Self Learning Math as a Hobby

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I took calculus as an elective the last year of my undergrad (biology major) and I enjoyed it more than I thought I would. I think all my high school math teachers were just garbage, because it made me remember how much I liked math as a kid. If I wanted to keep learning, mostly as a hobby, where should I go next? I suppose the obvious answer is to continue with calculus up to and including multivariable calculus, but I really don't know where to go from there. Linear algebra seems cool.