Most Helpful Girl
It's a beach festival, so definitely a cute bathing suit.
A sarong because you can cover up in case it get's a tiny bit chilly at night and you can style them into some kind of last minute dress.
I don't know how long you're staying in Portugal but I would suggest also bringing
a maxi skirt with a slit (they're cheap at F21).
light weight shorts. ie link
A nice light weight tank top/ boho styled tank
Some kind of sun hat (like a floppy hat)
If you really want to be cute
ankle bracelets and boho styled accessories make add spark to a beach outfit.