Where to begin & how to learn what to look for?

When I was young I use to collect wheat pennies, buffalo nickels, susan b,, etc. It was fun but I did it on my own with no real direction. Fastforward to now I think my young son might enjoy it. He likes hanging onto quarters and he also likes opening packs of collectible pokemon cards. I've lurked on hear a couple days now and cant help bit wonder where to start and how to avoid missing out on a rare needle in the haystack, ie how do you learn all the rares to look for? Is it more about knowing whats common and when its not those are the ones you check? I was thinking rolls of nickels maybe cool to focus on one domination and able to grab rolls in qty for not much tied up in cash. But perhaps there is a better unit to focus on due to potential finds. I would like something that will have some reasonable finds to keep his interest instead of long dry spells of common rolls. Anyway enough of my ranting, the long and short of it is, whats some recommendations starting out?