Using light colored containers, shading containers, double potting & filling the space between the containers with an absorbent material you can moisten daily to take advantage of evaporative cooling (the second container also shades the container you're growing in), are all good ways to help keep soil temps down. Bubble wrap slows the transfer of heat, but it also helps to keep heat IN, once the soil has warmed, so it would be less effective than any of the other tips I offered.

Partially burying your veggie containers or 'non-display' type containers would also be helpful because it would employ the earth to help keep soil temps down.

Al

Everything that Al said, plus painting containers to make them light-colored.
In some situations, you might also want to elevate your containers to allow
air circulation beneath them - especially if sitting on a hot deck/pavement/patio.

Josh