Each level has several common elements that you should be familiar with in order to achieve 100% completion.
The meat and potatoes of the game. You can't finish the game without completing each mission (or failing five times). Each level has seven missions that tie into the main storyline somehow. After completing the seven missions in a level, the next level is unlocked.
Each level has one bonus mission not related to the main plot. Bonus missions tend to be more difficult than the other missions in the level. Your reward for completing a bonus mission is usually a vehicle of some sort.
If you need to get a vehicle quickly, you can simply use any pay phone. Pay phones are free to use, and you can call up any of the vehicles you have unlocked. If you've completely destroyed one of your own vehicles, you can repair it at the pay phone for a price.
Wrenches are scattered throughout each level. Simply touch the wrench and your vehicle will instantly be repaired to full strength. Cool, huh?
There are various glass windows featuring Krusty and the Buzz Cola logo in the levels. Usually they mark off some kind of shortcut. If you need to get somewhere fast, look for Krusty glass.
There are three street races on each level run by the DMV. Race 1 is a time trial run by Milhouse. Your goal is to complete a certain number of laps within a given time. Race 2 is a circuit race run by Nelson. The circuit race makes you complete a certain number of laps, but this time you're actually racing against other vehicles -- usually the character and vehicle for the next level. (Homer has to race Bart's Honor Roller, Bart has to race Lisa's Malibu Stacy Car, and so on.) Race 3 is a checkpoint race run by Ralph, which requires you to race from point "A" to point "B" on a level. Your reward for completing all three street races is a new vehicle.
There are seven collector cards in each level. Each collector card features an artifact from the Simpsons cartoon. Collect all seven to unlock another race track for the multiplayer game. Collect all 49 cards in the game and you can unlock the Itchy and Scratchy film.
These are the strange cameras spying on Springfield. They will appear seemingly out of thin air whenever you approach one. Some of the wasps will just watch you, others will shoot electric shocks at you which will zap your coins away. Use your attack button to destroy them. As the levels progress, wasp cameras will gain shielding, move quicker, and shoot multiple bursts at you... watch out!
Each level is filled with various gags that you can activate for humorous effects, ranging from exploding Squishee machines to catapulting cats to self-destructing toys. They're required for 100% completion, so find them all!
Buzz Cola Crates
Scattered throughout every level are crates bearing the Buzz Cola logo, as well as vending machines that serve up the evil cola. Hit these crates and vending machines to make them spew out coins. Collect as many coins as you can in order to purchase vehicles and outfits.
Gil seems to have taken up a job as car salesman for this game. You can find him in each level with three vehicles available for you to buy. Some of the vehicles are of questionable utility, though some excellent cars such as the Police Car can be had from him.
There are five vehicles to be unlocked in each level. The first vehicle is given to you automatically as the character's default vehicle: Homer's Family Sedan, Bart's Honor Roller, and so on. You can buy two vehicles from Gil, unlock one by completing three street races, and unlock one by completing the level's bonus mission.
There are also random vehicles driving along the road just to get in your way. However, if you need a quick ride, you can jump into any of these vehicles as if it were your own. Most of these "throwaway" vehicles should not be used for missions, as their stats pale in comparison to the normal pay phone vehicles.
Each level also has a special hidden vehicle which cannot be called via pay phone. Most of the hidden vehicles are purely novelty items (such as the broken monorail in Level 2), but others are actually superior to your normal vehicles (like the knight Boat in level Level 3), or are useful for other purposes (such as the monster truck in Level 5). While fun to use, these vehicles cannot be called up from a pay phone, so take care not to destroy them.
Each level has various places of business where you can purchase different character outfits. Some outfits are required to complete certain missions; others are strictly for show. They are required for obtaining 100% completion.