What kind of work out do I need?

so I'm planning to work out only on week ends, because I'm an engineering student and I really want to focus on studying, lately I gained a little bit of weight, I'm not fat I'm still slim but I think if I keep gaining weight this way I'll be fat..so what kind of work out will just keep me fit if I just keep doing it only on week ends?
What kind of work out do I need?
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