Ask a roommate or visitor
Your Sim may ask any adult Sim in the house on a date. In the ask menu, you'll find an option to invite them Downtown. If they accept, the asker will proceed to the phone and call a cab.
Call a Friend
Your Sim may invite anyone he (or she) knows on a date by calling them. They can even call Townies that they may have met in their previous visits Downtown. If the Sim accepts, your sim will call a cab, and meet their date Downtown.
Call a Cab
You don't need a date to go Downtown. Your Sim could meet their mate while wondering around Downtown by themselves. When you decide to go Downtown, have the Sim use the phone to call a cab. You'll find the option under services.

