I recently had an overnight stay in Maleny as I was hoping to see a platypus. The boardwalk was quite pleasant, but a little confusing. If you start from the Showgrounds, it runs alongside the creek behind where the camping area is. It runs towards town, but pops out onto the road well before you get to the park, then starts again across the road down near the post office but from here it is more of a broad pathway than a boardwalk.
Apparently you can potentially see platypus anywhere along the creek, but there were three main areas where they seem to be easier to find. One is at the main viewing platform, one is at the little platform on Coral St, and the other is from the road bridge hear the Showgrounds.
I was lucky enough to see platypus at all three locations, but then I got extra-lucky and met the Platypus Whisperer (look him up on Facebook). He runs tours, and takes beautiful photos, and is very knowledgeable. He was very generous with his time and knowledge, so I imagine his tours would be very interesting.