Learn Spanish (Latin America): Para mejorar tu metabolismo, a lo mejor debes hidratarte más. - To improve your metabolism, maybe you should hydrate yourself more.

It is a notable exception! While many expressions of doubt trigger the subjunctive, the fixed phrase a lo mejor almost always takes the indicative in spoken Spanish. This is different from words like quizás, which often take the subjunctive, or phrases from your current lesson like de ahí que, which strictly require it.

Why use the infinitive mejorar here instead of para que mejores?

We use para + infinitive when there is no change of subject in the sentence. Since the person improving their metabolism and the person hydrating are the same (you), the infinitive is used. You only use para que + subjunctive when two different subjects are involved.

Is it possible to place the reflexive pronoun te somewhere else instead of attaching it to hidratarte?

Yes. Because you have a conjugated verb working with an infinitive (debes hidratar), you have two valid choices. You can either attach the pronoun to the end of the infinitive as shown here, or you can place it right before the conjugated verb: te debes hidratar.

Does a lo mejor literally translate to "at the best"?