Chemistry:
Win all three rounds of Volleyball. Shaun will give you access to Myroc's lab. Go to his lab and play.

Marbles:
To unlock the marbles game, go to the shop and buy the marbles. The next day, your mother will tell you to play with Ann. Find Ann and ask her to play.

Hidden custom parts:
Successfully complete story mode to unlock two hidden custom parts.

Volleyball:
Win all three rounds of marbles. Ann should say "You can use my stuff". Get the scuba gear, go to your room, and change into it. Then, talk to Shaun (the hiphop character). He will say that the bully kicked his ball in the lake. Go to the back of his house and swim to the island. Grab the ball. Then talk to Shaun, and play.

Chicken Herding:
One day your mother will ask you help with the farm chores. Talk to Suzy and she will tell you to do three chores. Do them to play the Chicken Herding game.

Lawnmowers:
Win all three rounds of Chemistry. Myroc will let you use his things. Get the gas can and go to the gas station. Gill the can and talk to Tanya. She will say she needs gas. Use your gas and she will let you play.

RC Racing:
Win all three rounds of Chicken Herding to get $5 for doing her chores every day. Save up $33.75 and buy a car. Go to the gas station and talk to the kid to play RC Racing.