Went the weekend before last, conveniently.
Kristiana does still exist Mex- the police cracked down pretty hard about 18 months ago but it had a pretty bad effect by all accounts, hard drug usage went up pretty significantly so it's been allowed to regrow (according to my tour guide anyway- I didn't see it before). Fun place to spend a bit of time...and money if you're that way inclined. I'm not, but it was still very interesting to me.
Copenhagen in general I really loved, really pleasant place to be. At the same I wouldn't say there was absolutely tons to do from a tourist point of view- I don't feel like I missed much and we only had two and a half days. Can recommend this free walking tour if you're on a bit of a tight budget, was very good indeed considering it was only tipping:
http://www.newcopenhagentours.com/If you do want to do Tivoli, there's a boat tour which I can't remember the name of (sorry, it's right near the Parliament building) that do a joint ticket of their boat ticket and entry to Tivoli which is cheaper than the price of an entrance ticket to Tivoli on it's own- bit odd, but definitely worth a go, not the best tour ever but for about -10 Krone you can't complain!