Translation:We are having lots of fun in Switzerland.
However, I doubt you say "We are having really fun" in English, just as we don't say "Meillä on paljon hauskaa" in Finnish. I tried to put "We are having a really fun time in Switzerland" (we don't say "Meillä on hauskaa aikaa" either) but it was marked incorrect as for now. (Reported)
It's called the vowel harmony. There are back vowels (a, o and u), front vowels (ä, ö and y) and middle vowels (e and i). Back vowels are formed in the back of the mouth etc. Back and front vowels can't be in the same word (except for the loan words for example labyrintti = labyrinth). But middle vowels can be with both of them. So, if the word has only back (and middle) vowels the ending will be -ssa because then the harmony won't break. For example Pariisi (a is a back vowel and i a middle vowel) -> Pariisissa. If the word has only front (and middle) OR only middle vowels the ending will be -ssä. For example Sveitsi (both e and i are middle vowels) -> Sveitsissä. But. If the word is Wales, it's written with -ssa, but pronounced with -ssä, because the pronunciation is as in English. It doesn't have any back vowels. (I don't think there are a lot of this kind of words)
So I understood from the comments in another exercise that if the location ended –i (Suomi) the –i became –e when –ssa was added: Suomi --> Suomessa. And yet Sveitsi keeps its –i to become Sveitsissä. Is this again a matter of vowel harmony, or just one of those case-by-case exceptions that we will need to pick up on the fly?