Just as others have mentioned here, the best things to add to an A1200 are:
8MB RAM/FPU/RTC card or 020/030 accelerator with at least 8MB RAM
4GB CF card on a CF-IDE adapter formatted with Workbench 3.1 on it
PCMCIA card reader, either SD or CF (remember SD cards are limited to 2GB each as SDHC isn't supported yet, but are cheaper than CF)
PCMCIA network card, either wireless or ethernet
CD32-compatible control pad or at least a Sega MasterSystem or MegaDrive/Genesis pad (which the Amiga uses as a 2-button pad)
Subway USB card
USB Missile Launcher
USB card reader
I have most of the stuff from Cammy's list...
GVP 030/40 "Typhoon" (with scsi) from softhut. The GVP also is equipped with a math coprocessor. I'm not sure which one. I ordered it with the 32mb ram chip. I did look at getting a 64mb FPM SIM but those seem to be quite rare. Also they look like much larger SIMMS.
NOS A1200,Subway and PCMCIA network board are scheduled to arrive from amigakit tomorrow. Even if that delivery slips a day, I'll still have it for the holiday weekend...
I'm also going with a 4GB CF HD adapter. I have 2 cards one is 133x and the other is 80x. I wonder if it makes a difference? I also have a 32GB card, but I'm holding off trying that immediately.
I did not actually order the USB rocket launcher yet, I really wanted to build (my own) upsized version that uses standard lawn-darts as ammunition.
I'm having a setback on that. Apparently, There is a real shortage of lawn-darts at the moment - even on the second hand market.