In addition to @Chronocidal's answer, you can also use nature to show the passage of time.
Even within the confines of a wagon with no outside view, you could have mice that are breeding. They can have up to 10 litters a year, so plenty of generations to be born, grow up and leave.
And if you can cast a glance outside through a slit in the wagon or something, you could see birds making nests, spring/summer/autumn plants starting to flower, trees getting or loosing leaves, dry/rainy seasons etc.
And on even longer time scales, if the group passes the same place multiple times, you can use things like trees growing and falling over, abandoned pastures slowly getting covered by forest, fires clearing a grassland/savanna and subsequent new growth coming back, etc.
Basically at every time scale you can find things in nature to change, because nature has generational cycles ranging from days, to centuries. Nature may not be the easiest to use for the shortest time scales though since they mostly cover small insects, fungi, etc that most people wouldn't normally notice.