Here is a suggestion for an ongoing competition (inspired by the billionth-bet competition):

Every bet whose ID is a multiple of 1,000,000 wins a bonus equal to the amount bet (maybe up to some maximum, or something like that).

I would expect that the amount given away should be negligible for the site: it already gives away 8 times the amount bet every 10,000 bets roughly, with the jackpot feature.
However, unlike jackpot, this would require some skill.

Regards.