Tips on disciplining a defiant toddler?

Hello, We have a beautiful 2.5 year old daughter and she is a great kid. The only issue we have is when she does need to be disciplined (which is not very often) she is not affected by any type of punishment. We are running out of ideas. When we first started to discipline with more than just a no, we would put her in timeouts where she had to sit down on the floor for a certain length of time (the time amount would match her age so right now because she is 2.5 she gets 2.5 minutes). A timeout on the floor worked for awhile but then she started thinking that it was funny and it became ineffective, we then moved to timeouts in a corner with the same time idea but that also soon became ineffective because she thinks it is hilarious and would put herself in timeouts after doing something she shouldn't of (like hit her friend) without us even telling her to go into timeout. We then went to putting her in her room with same time frame and shutting the door. she hated this kind of discipline because she was separated from others so it worked for awhile but she now again thinks this is funny and now when she is mad at us she says, "I go timeout" and goes to her room and slams the door. We are very consistent with discipline and feel like we have been from the start so I just don't know what to do next. Spanking is not an option for us even though it is tempting! Like I said, she is pretty well behaved but I feel like it is starting to go downhill because we have nothing to discourage bad behavior any longer. Do you guys have any other ideas on how to get through to her? Her main issues that I have to discipline for are hitting, trying to head butt her 1 month old brother, and not listening. Please help me!