Verb (type 3): to have fun, to amuse.
There is a bit of a play on words in this advertisement for the Särkänniemi amusement park. ”Kaikkea mitä huvitella voi” literally means something like ”Everything one can have fun (with).” There is a common phrase in Finnish, ”Kaikkea mitä kuvitella voi,” that means ”Everything one could imagine.” Here, instead of kuvitella, which means ”to imagine” we see huvitella.
Let’s translate the other words in the slogan:
Kaikkea is partitive of kaikki, which means ”everything,” ”everyone” or ”all.”
Voi is the 3rd person singular of voida, here meaning ”one could.”