Cheap Travel Tips

My bf's birthday is coming up and it's a big one, his 21st and golden birthday. I want to take him to Puerto Rico as a surprise gift. I need budget friendly tips for where to book flights, stays, rental car, and excursions. I was considering Frontier Airlines but I've heard mixed reviews about them, for 3 days, May 17-20 flying out of either Milwaukee WI or Chicago IL, (Chicago I would have to pay for airport parking around $50 and MKE I could park at a friends house and have them drop us off at the airport). Would I be fine with the personal item bag the flight comes with or should I add on a carry on or checked bag and we both share it? As far as a rental car, should I get an SUV or would a typical sedan be fine? and where is the best place to rent a vehicle since we are both under the age of 25? And then for excursions, I want to go zip lining over the Yunque rainforest and go either ATV or horseback riding, where are the best places to book those? I have been unemployed for 6 months but start a new job in a week so I'm not too worried about not being able to make the money back but I do want this trip to be budget friendly while still enjoyable, he will most likely want to pay for things while we are there anyways.