Regular trips to the public library -- check out age appropriate story/activity times.
Area parks
Coloring and cutting are good projects for home -- good for school skills
Scavenger/ Nature hunt through the neighborhood
Find something from nature and research it together on the Internet
Chores around the house -- you can set up obstacle courses w/ laundry baskets, cardboard boxes, -- bean bag tosses, stacking things
Play dates w/ preschool friends
Time him doing things and encourage him to beat his own times -- make a chart to show progress.
If you are near a safe beach, the sand can provide hours of free fun.
Cooking together -- find kid friendly recipes on Internet or cookbooks from the library
If you include the area in which you live, people might suggest specific places to visit that are free/cheap -- Orange county publishes newspapers/magazines re Parenting that list schedules of events.