How should I decorate a Spanish Epiphany cake?
If you do a google image search for roscón de reyes (Spanish spelling) you'll find them usually decorated with candied fruit, either just red (glacé cherries), red and green or red, green and orange. Sources vary, whether the fruit are supposed to stand for the jewels, the Three Wise Men brought to Bethlehem or for the jewels on a crown per se. Coarse sugar and chopped / slivered almonds are also rather frequent. Overall, there seem lots of different layouts and decorations, so I think, if you somewhat stick to "(alternating types of) candied fruit arranged on top", you're good to go. 

If you do have a bit of a perfectionistic attitude, remember that the traditional decoration may vary from region to region, even from family to family. While I personally believe that your friends are probably already deeply moved by your gesture (otherwise I'd have them bring the cake), if you want some inspiration, I suggest you take a walk and check the window displays of your local bakeries - they'll have what's considered "the right" version in your place.