Building/Photography Question

IÂ've been taking pictures for several years of Downtown Chicago, and one thing IÂ've always wanted to do was shoot from up top different buildings like you see in a Photographic tour of Chicago and other books. My dad says that there are buildings you can go into, but neither he nor I know which ones those areÂ… I saw a thread recently where someone shot from the 75th floor of the AON building, can you just walk in? My parents say you need to be a licensed photographer and have written permissionÂ… So I ask the more knowledgeable people, What buildings if any can I go into and shoot from, cause the AON Building looks really coolÂ…>