Smack dab in the middle of Oakland, even in the middle of summer, there's a big slab of ice. Seven days a week, you're likely to find kids and adults donning pairs of ice skates, and either whipping around the rink at dizzying speeds, or clinging for dear life to the sides, just trying to catch their balance.
You can pay a low flat fee (I think it was under $10/per person for adults), and skate around all afternoon. Or, you can practice or play hockey on their second rink. Lessons are available for those who would like to learn to figure skate, or maybe even just do basic skating without landing on their back sides.
The rink is located a block away from the 19th Street BART station, making it very convenient to get to. But, if you prefer to drive, there is parking right across the street.