Each evening before bed, I set out three trays with a toy he rarely chooses, a coloring page, an independent activity, a puzzle, playdough, dot marker page, etc. on each one. He can work on these trays at any time during the day and when I notice he's bored I will remind him to go "pick out a tray." I plan to add more trays as we go along.
This idea is adapted from 1+1+1=1. Her system is more involved, educational and thought out. Mine basically keeps S occupied while I unload the dishwasher ;)