Learn Russian: Это самый гладкий асфальт в городе, здесь приятно кататься на самокате. - This is the smoothest asphalt in the city; it is nice to ride a scooter here.

кататься is the multidirectional verb. In this sentence, riding a scooter is a general, continuous activity done for pleasure around the area (здесь), not a single trip from point A to point B. If you were coasting down a specific hill right now, you would use катиться.

The lesson is about the transitive verbs катить and катать. Why does this verb have a -ся ending?

катить and катать mean to roll or drive something else (like pushing a wheelbarrow or giving someone a ride). To talk about riding a vehicle yourself for fun or transport (like a scooter, bike, or sled), you add the reflexive suffix -ся.

Why is it translated as на самокате (Prepositional case) rather than using the Instrumental case for the means of transport?

In Russian, to express the mode of transport you are riding or taking, you almost always use на + Prepositional case (like на автобусе or на поезде). This is especially true for things you ride on top of, like a bicycle, a horse, or a scooter (самокат).

Is it possible to use a single-word superlative suffix instead of самый гладкий?