Dumbbells - two pound dumbbells, to be exact. Your Wiimote is waiting to pump (clap) you up with this $20 "peripheral" available from Everlast, said to be compatible with a slew of exergaming titles.
The listing says the weights don't interfere with the IR capability so, hell, why not hook it up to Dead Space: Extraction or Madworld for a real workout?
Wii Weights (2lb Dumbbells) [Everlast, thanks fusioncam]