After doing the nonsense words and made up words investigations, you can move on to real words. The goal here is simply to break words into their meaningful parts, their morphemes. You can do this, of course, with any category, but it will be easier to limit the focus at this point on nouns, and the kinds of suffixes that nouns take, so it’s probably best to offer a list rather than have the students generate their own words, at least at first. Here are some ideas. You can use others (that may related to topics you’re covering in your classrooms).
cats = cat + s
probability = probabil + ity (probable)
freedom = free + dom
happiness = happi (happy) + ness
wreckage = wreck + age
runner = runn (run) + er
Some of these examples illustrate how spelling can change with the addition of some of the affixes (affix is a cover term for suffix and prefix), so you can decide if you want to detour here to focus on spelling. There are lots of interesting spelling detours to take, actually, so I’ll have a separate post about some of them and some of the many excellent resources.
This lesson on TeachLing is good, but it's on breaking words of all different parts of speech into their morphemes, so I wouldn’t do it here if you’re trying to limit the scope to just nouns. It might, however, be useful as a model.