Planetary protection rules could be a reason not to bring biofilms to Mars, but because several landers and satellites that crashed or landed on Mars were probably not sterilized these rules were broken already many times.
Nostoc Commune is a species of cyanobacteria that develop a biofilm that serves as a protection against harsh conditions.
They use the energy of light to synthesize organic compounds from CO$_2$ and produce O$_2$ this way.
Because of this biofilm Nostoc Commune bacteria can survive high levels of UV radiation, are resistant to heat and repeated patterns of freezing and thawing and can survive long periods without water.
So why not place these bacteria with their biofilm in a simple plastic cover on a clay-rich, lowland region near the equator so that they are only protected against the dusty winds ?
With a camera to take only one image a day it would be very interesting to see if they can survive the harsh conditions on this planet.