One example which is actually relatively common in everyday life, especially for people who do a lot of traveling (e.g., backpackers and some business people), is the knapsack problem. The applications section gives several other fairly common situations where this problem is encountered. The Wikipedia article also lists a few methods that can be used to determine a solution, but especially for relatively simple cases, something like the greedy approximation algorithm can be implemented fairly easily, and often works relatively well.
Nonetheless, as that algorithm section states, a basic greedy algorithm may sometimes be far from optimal. I don't know of any examples offhand, but I suspect it should not be overly difficult to construct (or perhaps find online) a relatively simple one that would help to illustrate this point to your students.