Nerby the river: Vltava
The plain of Letná , and especially its space around former Stalin Monument , was probably the first place, where everybody, who had wheels or little wheels, raged.
In 2006 the reconstruction of roads was made in Letná, so the surface for skating is of better quality . However, the roads are zigzaged and are quite narrow . This is, owing to relatively great visits of this place., rather disadvantage. Anyway, this moderate uncomfortability will be substituted with beautiful view to Prague. It is also important that some snug garden-restaurants are available , where you can obtain good refreshment ( provided you are tired or order some cold or hot drinks).
Information and warnings on travelling abroad is to be found HERE.