one of them gun holder thingys should come in real handy. I like the holster made by bladetech, and F.I.S.T, some like the Fobus holsters which are quite inexpensive (as is the glock holster). Of course you can get leather from rosen, kramer, sparks etc.
And definately dump the plastic sights and get some decent steel ones with or without tritium inserts. Meprolight, novak, MMC, heine are some good ones that come to mind.
I presume you have a decent pair of glasses and ear protection, if not get them. And you may want to snap up some hicap mags, at least get a couple of spare 10 rounders.
Finally unless you have really big hands after a while you may find grippin the 21 is like holding a 2x4. There are several people that do grip reductions. I had mine done by dane burns and really like it tho it didn't do much for the looks.
And lots and lots of ammo.