Dried apricots, pref. unsulphured.
Can be dumped in a jersey pocket as they are, so no wrappers to deal with or dispose of on the ride. Easy to chew and swallow.
Moderately low GI (google he say GI=31, GL=8) so best munched steadily before you bonk, but enough readily available sugar to bring you back from the brink reasonably fast if you do.