Answer
The period of oscillation is shorter when you stand (rather than sit) on a playground swing.
Work Step by Step
Some internet research about pendulums, or referring to Footnote 2 on page 358, shows that the period $T = 2 \pi \sqrt{\frac{L}{g}}$.
When you stand on the swing, the pendulum's effective length L is reduced because you have moved the pendulum bob (your body) closer to the pivot.
This decreases T, the period.
