(Sound) Echo from X takes 5-7 days,
If there would be air at room temperature between Earth and Moon, sound would take about 28 days from Earth to Moon and echoed back, with helium instead of air 9 days, and with hydrogen about 7 days.

Calculated from http://www.phy.mtu.edu/~suits/SpeedofSoundOther.html and http://en.wikipedia.org/wiki/Moon

You could also reflect Earth light from Pluto back to Earth, 11 times this double Earth - Pluto distance to get about 5 days of travel of light, much depending of the orbital position of Pluto.