Executive summary: parking is available in the lot right outside the door. Restrooms are a bit cramped but well kept. Thai Pavilion is AMAZING...go see it!
I am a huge fan of Olbrich Gardens. Whether I'm looking for a place to walk on a beautiful summer's day, or trying to hone my camera skills by capturing that perfect bloom with just the right focus and lighting, or looking to escape a cold and dreary winter in the always warm and colorful Bolz Conservatory this place fits the bill!
The Thai Pavilion is my favorite place in the park. When it's sunny, the gold leaf and mirrored tiles glow. When it's overcast, you can really focus on the details without being blinded by the shine!
Outdoor gardens are free and Bolz Conservatory costs $2 or more depending on special events. That's not a lot to pay to keep this beautiful place operating, imho. If you're a true fan, consider purchasing a membership. I hear they offer discounts at their sister garden: Phoenix Botanical Garden.