Sounds like a fun event. I love loosely organized things like that. Here are some ideas:
Using playing cards as tokens can make it a 'poker run'. Cards in sealed envelopes, extra prizes for best hand, worst hand, etc.
Maybe a simple compass heading at each mark to the next one.
Possibly include info at each mark necessary for the next mark to ensure the order is correct; For example: Heading 300 degrees, card from BLUE envelope (and the mark has 4 envelopes to choose from)
Consider setting up marks, and giving out maps. But no course info until just before the start. Or just hand them out at the skipper's meeting, and say 'GO!" for a LeMans start.
Maybe have the marks at the corners of the lake, with the instructions to round them all in any order... 3BF style.
There used to be an event at Klamath Lake Oregon, called 'The Battle of Trafalgar'. Boats are divided into 3 fleets, British, French and Spanish, with appropriate flags. You carry a cocktail napkin in your rigging for each crew member you would normally have (held with thread), and are allowed any device you want to fill and launch water balloons. When your napkins are gone, you are 'sunk'. Could be fun on a hot Texas lake.