The "agua mineral" is a very nice park with a relaxed environment. Besides the crystal clean…read morenatural fresh water flowing in the swimming pools, there are 2 very interesting trails in the "cerrado" environment, originally the second biggest forest area in Brazil after Amazone. There you will have a great opportunity of enjoying silence and contact with nature, as hikers are few and far between even on the busiest days. Go for the hikes, visit the meditation area, where you can see the uniqueness of the Cerrado, a kind of savanah of Brazil central, and after that refresh yourself in the large pools.
The 2 trails: Capivara (about 15-20 minutes) and Cristal Água (1 hour). They're both really easy. Capivara is well shadowed and closer to the pool area, and it's quite busy at all times so no special precautions are needed.
Bad points: Foreigners get charged 15 Real as opposed to 7-8 Real for locals. It's a pity they don't have bike rentals for tourists. And yes, for access you need a car, as the park is badly served by public transport.
Tips: Avoid coming by taxi, and if possible, let a brazilian take you. Bring comfortable shoes, a bottle of water, sunscreen and snacks. It's great for kids, specially to observe the little funny monkeys, but take care with your food, they are smart enough to catch it and make you laugh.