In French, we say je regarde le paysage because there is only one paysage to see. We do not say je regarde un paysage unless maybe if you are talking about paintings1.
When referring to a trip, you can still use the singular:
En faisant du vélo, je regardais le paysage.
It would work even if the paysage changed but if you want to explicitly say that you saw various landscapes, je regardais les paysages (variés qui s'offraient à mes yeux) would indeed be the idiomatic way.
Des paysages is possible if you are talking about specific landscapes:
En faisant du vélo, je voyais défiler des paysages familiers.
1 You can also use un paysage if what this paysage is unspecified, e.g. Quand je regarde un paysage, je ne peux m'empêcher d'en voir tous les défauts ("When I look at a landscape I cannot help seeing all its defects." Oscar Wilde)