A tip about the battery: I'd replace it anyway even if it hasn't leaked yet. Since it has sat so long, now that you are energizing it it may start to leak. It really depends upon the type of battery of course, but imho if it's one of the barrel-shaped NiCds don't trust it.
As for upgrades/fun, I recommend you get a sidecar-type harddrive/memory controller. Next, you could get an internal rom switcher and a later rom (2.04 or 3.1). You could upgrade the Agnus to a 1MB variety (8372?) and have 1MB chipram with the appropriate m/b hack (see Aminet). If you're real lucky, you'll find a 68020 or 68030 accelerator that plugs into the original 68000 socket inside the case. There is also an Amiga.org user here (hppacito) who is working on making a new 68020 accelerator for the A500/600. For faster serial, Jens Schoenfeld (
www.amiga.ga) sells an A500 clockport adapter (a la the A1200) so you could plug in a fast serial silversurfer or other clockport card (USB?) in the old 500. afaik, the only thing really difficult to add to an a500 is an rtg graphics card (there is even an internal flickerfixer vga adaptor made by ICD .....FlickerFreeVideo or suchlike).
