Great to hear you enjoyed the first part. Don't worry, there's lots more to discover.
You mean the 'old key'? You'll need to do the mini quest of the lost guy, Mr Ryan, before you can find what that unlocks. By the way, even his name is a very subtle hint.
Combat? What combat? It's a peaceful walk in the park... or is it? ;)
But just in case... the place where you use the key will give you a few hints about where to look. Also, listening near the statue in the middle of the park isn't a complete waste of time. To be fair, there is something that is not visually obvious and (very?) hard to find. Just make sure you're searching in the right area. Have fun!