You'll start the game in the overworld, where you'll see the name of your current area surrounded by four possible destinations. Click one (duh). The options with colorful backgrounds take you to other areas of Wackyland, while the options that look like links (they are links, in fact) take you to Wackyland's silly spots where you'll find something to read that will hopefully amuse you. (Exception: The "Portal to Realms Still Stranger" is not a silly spot, but rather the way you get back to the main menu.)
After you're done enjoying a silly spot, click Leave to return to the overworld. Some silly spots offer you some form of further amusement; if you would like to partake in it, you should instead click Accept.
Most silly spots will change, depending on which day of the week is today. This of course means that you can come back on a different day to find more silliness. (You can also just change your computer's clock and re-enter Wackyland, you dirty cheater, you.)
A method of exercise in the real world, a form of convenience in Wackyland. Both should only be attempted outdoors.
While in the overworld, you can press the arrow keys to move in the direction pressed. Well, actually, pressing Down doesn't actually send you burrowing underground, it sends you south. You know what I mean.
So, you've found something really funny in Wackyland, and you wish you had a version of that silliness that was always there, no matter what day of the week it was, and you could easily share it with your friends. Fear not, for that's what the Snapshots feature is for! Just click the Snapshot link (in the same part of the page as Leave and Accept) to get a never-changing page featuring the same silliness you just saw and a handful of sharing tools. As an added bonus, Snapshot pages don't require JavaScript, so even that paranoid friend who keeps JavaScript turned off and lives a thousand miles from every government building can enjoy the looniness of Wackyland! (Exception: Snapshots of Wacky Theater still need JavaScript. Nothin' I can do about that.)
"It's great that Wackyland is a little different every day, but how am I going to know when I've seen everything??" Never fear, for the game will keep a Checklist of what you've discovered as you explore Wackyland, and you can click "View Checklist" on the main menu to see it! Of course, viewing the Checklist will reveal which parts of Wackyland change each day, which could be considered spoiler-riffic, so only check it out if you don't mind that. Note that if a spot offers you something and you haven't clicked Accept, it won't get that mark on the Checklist until you do.