Toshimaen amusement park is in....Toshimaen. It's on the Toei Oedo subway line...easy to get to from Shinjuku.
I don't know about any other events in March...but if I find out, I'll tell you!
Oh! There is one Star Wars-only event on March 5th my friend told me about. I can't make it, but I can find out if anyone's interested...