First, you have a key decision:
Do you pitch to your own team, or to the other team?
Pitching to the other team becomes a TOTAL cluster****, but is more fun for the "good" players - teams that have played before and have a real pitcher will asscrush everybody else. Like, a pickup team has literally zero shot to win. Unless you have a ton or zero good players, it's probably better to pitch to your own team (for a fund raiser, this will be MUCH more fun/successful for everybody).
From there, play w/ teams of 5 or 6 (six is great for coed), put out a snow fence at something like 125-150', and assign distances for each hit - past a fielder/pitcher is a single, past the 'infield' line in the air is a double, off the fence is a triple, over the fence is a homer. No baserunners, they advance if forced.
No walks, and something like 3-pitch max. No "count" but put up a strike zone dummy behind the batter, and if it hits that with no swing, the batter's out. From there, play something like 5 innings to keep things moving, and offer plenty of eating and drinking opportunities - there are a couple big charity events like this around here that use this system, and they're wildly successful.
(just reading jerz's, it's pretty similar - both seem easy to do on regulation SB fields and requires pretty minimal setup)