If swimming is your thing and you're here in the summer then the Nord Swimming Complex is an open-air centre with a couple of pools. One seems to be for the serious swimmers, another for diving and a third fun one with a water chute.
This is located in the Mladezhki Park, to the east of the city, and can be accessed from the riverside walk.
Equipment: I assume you need your own costume and towel.
Ruse Tennis Club is located in the Mladezhki Park, heading east from the city centre. This seems to be sponsored by the Zagorka brewery - need I say anything further?