ContiTube MTB 29 x 1.75-2.5 PV 42mm Presta

You never know for sure when you'll have to put in a fresh tube, but odds are it will be as soon as you go for a ride without one. Save yourself the trouble of walking home by picking up a spare Continental MTB 29in Tube, which will fit easily into a pack or pocket so you'll be able to save yourself in case of a blowout.