C.B.
My son (2 1/2) is the same way and we are planning on homeschooling so I've been gathering information to use, too. There's definitely a time factor on your end involved but I don't think you have to be crazy organized - especially at this age. I have a basket where I keep things I've printed out, dry erase markers, easy readers, etc. so when he asks to "do school" I just grab it and lay everything out.
Some of our favorites: Do A Dot markers. Traceable letters (You can find tons online to print off, but buy some page protectors and dry erase markers and you won't have to print over and over again - just print one and put it in the protector and use dry erase markers on it! They wipe right off!). Activity workbooks.
We love Starfall! There are a lot of free activities that we've been using for a long time, but since he started getting kind of bored with them I went ahead and subscribed for the year - now he has access to geometry, multiplication/division, lots of reading games, etc. He loves it! Not to mention, he's mastering some basic computer skills!
Also, PBSkids.org has a lot of good educational/fun games that are all free.
We made flashcards of sight words using this list: http://www.k12reader.com/dolch-word-list-sorted-by-freque... and he is having tons of fun playing with them. Within 3 days he had the first 13 words memorized and he read his first sentence from a Step 1 reader! (For ages 4-7.)
My other advice would just be to scour homeschooling blogs, specifically ones that post lots of things about activities for 4/5 year olds and don't let things (or people) hold you back just because certain things are recommended for kids older than your son. Follow his lead/interests.
Some good blogs are:
http://1plus1plus1equals1.blogspot.com/
Tot School: http://www.confessionsofahomeschooler.com/blog/tag/teeny-...
Tons others that I can't think of right now... just search - you'll find them!