Have a skirt for the float that matches the auto repair sign colors (if there are any major colors). Instead of spending alot on fancy plastic skirts what you can do is go to any party store that has plastic table cloth on a roll (usually comes in 50-100 ft) and buy that, bartzs has it for $15 or so and comes in a lot of colors.
I do like the idea for the large tools, but instead of just painting them in the basic matching color use matalic or glitter paint.
Have late 60's music playing from a boom box or sound system.
You can always add ballons for an added pop in the colors of the repair shop but maybe mix in a few mettalic ones, that will grab attention.
Could do stremers drapped around the cars, like they do to bikes. Tie onto the mirrors and wheels, they will move with the wind or the float to grab attention.
Are you allowed to throw candy... that will add some pow because a lot of people like free candy. If you are handing out a flyer with a discount coupon... maybe tape a sucker or tootsie roll to the flyer, at least then people will look at the flyer to take the candy off, oh and look a coupon.
It seems that in 1968 a lot of float pictures I find were decorated with peace signs, red white and blue colors, and happy people riding them.