While it is easy to walk about central Stockholm, don't be afraid to use the subway! It is quite easy to figure out with only three lines (red, green and blue). Subway stations are spread throughout the city and easy to spot... just look for the sign with the white circle and blue T. When buying tickets, it is important to consider how much/often you will be traveling. It is more economical to buy a ticket strip or day pass (available for one or three day periods) than a one time ticket. Combining subway travel with buses is simple as the tickets and passes can used for both! Some commuter boats can be used with the day passes (not one time tickets or strips). It is easiest to buy tickets at the Pressbyrån kiosks located at or near subway stations. There you can use either cash or a credit card. All travel on subways, commuter trains, buses and certain boats is included in the Stockholm Card.
Rival Hotel is Mariatorget (red line), just a 2-3 minute walk from the hotel. To give you an example of how convenient the subway is... a trip from Mariatorget to Östermalmstorg, on the other side of the city, just takes 10 minutes with the subway.