1- Context might help. I hadn't noticed this problem before.
2- C nouns can be used in the singular. The problem with 'activity' is that some nouns can be both C & U according to context. If you are thinking about work, then it's uncountable, but if you are thinking about a tesk, or piece of work, then it is countable. Rather than labelling a noun as one or the other, think of U & C as characteristics they have, which can be different according to context:
I like coffee (substance/ drink U)
I'd like a coffee (unit of drink- C)