How do you get the secret achievements in Valet Hero?

Solution 1:

I don't have all of them yet, so I'm going to have to come back and edit this answer to include other ones, but here are the ones I have so far:

• Sunday Drive (5): Drive 50 miles.
• Road Trip (10): Drive 250 miles.
• Out of Gas (5): Take a full tank of gas to empty, in one go.
• Refueled (5): Let you gas meter fill up, to full, from empty.
• Super Sized (20): Achieve a combo level of 15 on any gold challenge.
• Bad Driver (5): Crash 100 times throughout your Valet Hero career. (Challenges only.)

Six down, seven to go.

These are pretty self-explanatory, although I think Super Sized is glitched. I got it (as well as the normal achievements Value Meal and Drive Thru) on silver challenges.