In this variation, the stumps are replaced by whatever is handy - usually a rubbish bin, or an empty esky1 turned onto its end and, instead of a cricket ball, a tennis ball is used. This is a safety measure in case you happen to hit it into a group of unwitting picnickers.
There are no restrictions on player numbers - in fact, the more the better, and there is no division into teams. Basically, two people are chosen to bat, and the rest are fielders until a batter gets out. The person who got the batter out then becomes the new batter. Getting out is a purely arbitrary thing however, given that there is never an umpire, and local rules may apply.
There are various local rules that can be used:
The batters must run every time they hit the ball no matter how far the ball has gone.
A rule that can be applied dependent on the local environment. This might be 'over the fence and you're out' or 'over the break and you're out'2, or if the fielders are sick of any particular batter, they could just declare that 'over five minutes and you're out'.
Most importantly, no score is kept. That's the beauty of beach cricket.
Traditionally played during summer and especially on public holidays or at large family picnics, beach cricket is best played with a beer in your hand and zinc cream on your nose. A couple of other compulsory elements also apply. Namely that somebody should commentate wildly throughout the play3; that there is a fielder at deep mid-on4; and that the ball be stolen by a dog who is then chased by at least half of the players, usually to no avail. It should also be expected that a meticulously crafted sandcastle will be completely destroyed while a fielder attempts a particularly spectacular catch leaving several weeping children, and that someone's father5 will hog the batting and refuse to go out.