My brother and I often have a Slurpee bet when we play. Loser buys the Slurpees on the way home. I usually spot him a four stroke per 18/hole handicap.
He's rated ~890, I'm rated ~970, but I play worse in casual rounds than in tourneys, and he plays better in casual than in tourneys, so 4 strokes is about right when we play with just the two of us.
Slurpees are less than $1.50 each, so it's a minor bet -- stays fun. More $ than that and I would feel bad winning... good feelings are more important than side bets.
With a lot of friends, we'll have a $X per nine bet and an overall score bet... so if you want to keep it cheap for 36 holes, everyone throws in $5 or $10 or whatever... $1 or $2 per nine, plus $1 or $2 for the overall score.
and no one is ever out of it, since every nine holes, you start anew. And then with the overall score, someone can lose every nine, but still win the overall if they just play consistent...