For instance, would you use: der Snack, der Imbiss, die Kleinigkeit, der Schnellimbiss, der Happen, etc.?
And when would you use which term? i.e. for what kind of snack — a snack commercially packaged or one provided at an office?
Imbiss, and especially Schnellimbiss, is more likely to be understood as food stall when no context is given.
Both words can be applied to the junk food, but as you can guess from the discussion in comments below @HansAdler's answer, this can lead to funny sentences.
Ich esse einen Imbiss.
is correct and will be understood and, though, it sounds rather like a monster that devours a snack stall as @ORMapper mentions in his comment.
In my opinion, Snack and Kleinigkeit are more appropriate and the words that are used most often nowadays.
Happen is indeed used, too; but less frequently. Actually, I associate this word rather with a different context. Consider a mum that is worried about her child that is not eating enough. She might say to her child that they should eat "noch einen Happen".
Long story short: Snack and Kleinigkeit are the words I'd go with.
The best all-round translation is Imbiss. All the others only fit in certain situations or are used only in some regions, but I cannot immediately think of a context for snack in which Imbiss doesn't fit. An alternative that also fits in many cases is Zwischenmahlzeit (in-between meal). It's not as formal as it might appear to English speakers, but it has some semantical restrictions that should be obvious.
As Jan pointed out in a comment, snack can refer to potato crisps, salt sticks, pop corn etc., whereas Imbiss cannot because it always refers to something more substantial. So in German you have to say something like Knabberzeug for everything that doesn't fall on the sandwich side of a dividing line between potato crisps and sandwiches that's roughly in the area of cereal bars.
I think this linguistic difference either reflects different attitudes to food by the different speaker communities, or maybe even causes them. I was quite shocked when I discovered that my daughter's otherwise excellent English nursery served potato crisps as part of a meal (during an excursion). Years earlier I had already been shocked when I first saw an English adult eat crisps for lunch on a bus. These culinary practices are almost unthinkable in most of Europe. (I confess that living in the UK for a few years partially converted me.)