For example, what is the difference between ゆっくり and ゆっくりと?
Well, there is not much difference between the two. Using the と in this instance really just "Emphasizes" ゆっくり. It is hard to quantify in English, but sort of has the feeling of trying to make the preceding more clearer in the emphatical sense.
Eg, these two statements are almost identical in meaning "Study Hard", but the bottom one just has that extra little bit of meaning, like you would say in English "Study really hard".