Well, most sentences like "On je kanapkę" make a lot more sense in Present Continuous, Present Simple "He eats a sandwich" is accepted, but rather strange. In general Present Continuous seems to make sense more often until words like "always", "usually" etc. are introduced.
But 99% of the Polish verbs (the exceptions are verbs of motion, like to go/to be going) work both for Present Simple and Present Continuous. So "Lew pije" is "[The/A] lion [drinks/is drinking]".
There are also several so-called habitual verbs (for "to drink" it's "pijać") which would work only for Present Simple, but they are not in scope of this course and you can easily use the basic ones.