Colonizing Mars usually conjures up one of two ideas: life lived in large transparent domes, or some how terraforming mars with out massively altering the planet. The possible necessity of the first, and the difficulties in the second, arise from mars be small and cold. Which is related to how small it is as well. This combination has also left Mars with no magnetosphere to protect from harmful radiation.
So I've given some thought to a relatively radical idea on how to improve Mars in the long term, and I have some ideas on massive terraforming that would require no one be situated on the surface of mars while in progress.
You send crews out to the asteroid belt, and start aiming asteroids towards Mars (possibly through attaching small ion drives, possibly via a single push with a craft, many options are available). And you let these big space rocks hit Mars.
This is a fairly massive solar system engineering project, you are trying to dump a good chunck of the asteroid belt onto Mars, enough to build up it's gravity to a decent size. And in the process, possibly heating it up a fair bit (that's a lot of kinetic energy being transferred on each hit). This is a some what slow, and possibly rather expensive, project. Costs can be mitigated a little by doing the least work required to set a collision course. But that generally means it takes longer for said asteroid to get to Mars.
I can imagine a lot of people thinking of this as not very practical, but I believe I am simply looking at the longer term issues here. Yes, there is a small possibility we cna reheat mars's surface enough to get an atmosphere going again etc, but it is so small that any human-habitable atmosphere will begin leaking off, and that still doesn't help with the radiation problem.
The asteroid bombardment would fix the gravity based issues, supply more water, and possibly, maybe, give Mars enough energy to warm up its core and produce a magnetosphere once more. I don't know the numbers and formulas that need massive crunching to figure out the probability of this happening.
Of course, by the time we are in a technological position to perform this terraforming, we'd have had a chance to explore mars more, learned most of what we can, and gone deep enough to be sure that microbial or bigger life didn't simply follow Mars's fading warmth down deep into the crust & mantle. There may yet be (probably rather small) things living deep, deep underground.