By recommend, I mean content you actually find to be high quality, well done, and easy to absorb and follow. By relearn, I mean I have forgotten everything I ever learned in high school.
Formalizing e.g. limit is quite interesting! Limit is related to tendency; sequence x_n converging to x means for large enough n, x_n is sufficiently close to x. That is, you can choose N such that | x_n - x | < eps for n >= N. In some sense, you are concretely defining what rough terms mean!
To look into these, you can read through books disregarding proofs. While proofs do hold ideas, they can be headache-inducing.