You don't need a passport and it'll be in the 90s. Keep in mind it'll be in/around hurricane season... the south side of the island is generally more mild. I went last August and had a great time. Stayed with the GF in Yauco (near Ponce... opposite side of the island from San Juan).
Had a couple thunderstorms that lasted all night, but when you've got an airconditioner going as well you tend not to notice. If you're staying in a bigger city (San Juan) be careful about walking around outside at night. And be sure to check out Old San Juan and buy some passionfruit (parcha) sorbet (they call it something else, i forget) from the sidewalk vendors. That sh1t is DELICIOUS.