Physics, Chemistry and Biology! I really regret only studying physics on its own. I wish I did at least Chemistry. They are all fascinating subjects and will give you more options when its comes to specialising during your science degree
In addition to the “hard sciences” it is really important to learn how to write well! I can’t stress this enough. You have to write research papers about your work, and if they are not written well, no one will want to read them. Personally I think science is a creative process, so taking classes that stimulate your imagination are great. Problem solving is another skill to work on. Science requires a specific way of thinking, to distill ideas down, like taking a watery soup and boiling it down until it is rich and creamy with information goodness. You have to analyse problems in abstract ways and keep and unbiased, open mind about them.