Firstly, You're going to need to own a Highrise and an RC Orb. You can buy the Highrise at Horizon Condos in the furniture store. The RC Orb is the 500k milestone for playing Ball Race. It's hard to earn experience points when playing solo, so I recommend doing this with a friend so you can get rewards from melons.

Once you get the Highrise and the RC Orb, open the condo and put your RC orb into one of your inventory slots. You can then simply jump off the rail of the condo and continue falling until you reach the ground level and streets.

Once you get into the streets, you'll find its pretty much just misaligned roads and floating lights that are meant to be cars. But there is a lake you can hover over, or if you're up for it you can take a stroll in the park. You can also walk along the streets and explore the multiple buildings or race the fake light cars until you reach the map boundaries.