Mobile car wash
The problems with these are that you live and die by weather. Also, here in the South, they are both seasonal and you MUST diversify.
Lawn service is probably the most competitive small business in the U.S. You will never do it differently, only cheaper or more of it. The most successful lawn services I know buy other lawn service's out, including their contracts. The good thing is a bad thing, lawn service is extreme physical labor so there are always guys/companies wanting to get out of it.
Mobile detailing has been a disaster for anyone I know thats done it. There are hardly ever any contracts and it is a lot of leg work cold-calling or door-to-door selling. A lot of companies are now considering this solicitation and prevent it. Also, a vast majority of the population out there view detailing as overpriced. For those who do value detailing... its sub par. As a car guy, I think there is no way a mobile detail is equal to an in-shop, garaged, controlled environment detail job. IMO, the only way to get into mobile detailing is via mobile mechanic work with fleets (contracts).