fumado

This is a genuine build, and one of the clearest examples of what -ad- is for. Fumi is the verb. Fumo would be the smoke itself, the grey stuff coming off the fire. Fumado is neither: it is the activity, the going-on-ness of the verb, and that is exactly the job the aspect suffix does. Kanti is to sing, kantado is singing as a sustained activity; fumi is to smoke, fumado is smoking.

The gloss in most word lists says "smoking (habit)", and that undersells it. -Ad- marks duration or repetition, so fumado covers the habit a person has and also the plain ongoing act happening right now. La fumado en la ĉambro ĝenis min is about the smoke being made this evening, not about anybody's twenty-a-day. Both readings are the same word doing the same thing.

Two practical notes. The sign on the wall is usually Fumado malpermesita or Ne fumu, and the first is a noun precisely because a prohibition applies to the activity rather than to any one puff. And the -ad- noun always keeps the verb's argument structure, so you can say la fumado de cigaredoj in exactly the way English says "the smoking of cigarettes".