"If human beings are still biological creatures by the time we start
really colonizing space, then we should consider making other planets
more Earth-like.

My other suggestion is - if people still want to be organic as judging
by the reactions from film reviewers to the all-machine world of Cars -
to modify humans genetically so they can survive on chosen worlds.
Might be just a bit easier and cheaper than transforming an entire
planet to suit us."

Yes, but there need not be much need of genetic alteration- or drugs even possibly diet alteration.

Let's talk about Mars [cause I generally spent most of my time talking about the Moon:]. Mars' 1/3 of gravity may or may not be a problem. It's quite possible that instead of Maritans being weaker and unsuited to a 1 gee, they could end up on average being more physical fit because their lifestyle and/or normal way doing things requires more use of muscles. Instead skate boarding or walking, they might fly. Or if their normal way to get from Point A to Point B involves 3 gee acceleration and if they doing this daily or weekly, this could make them quite capable of living on Earth, if they chose- though might find the 1 gee environment "limiting" and boring.
The radiation problem in regards to Mars is *mostly* getting to Mars. As for living on the surface- most people living on Planet Earth who not living in 3rd world countries- don't spend much time living on surface- there are in buildings or vehicles. They might walk on sidewalks and go to parks- some might go camping- but few "live" camping.
Considering that Mars' atmosphere requires a spacesuit- Maritian are unlikely to spend much time camping. Instead you will have enclosed parks and domed cities- and these can be easily made so radiation isn't a problem. Oh, also radiation isn't uniform- if you going somewhere in daytime, the Sun will be most of radiation problem- and that is predictable and one can adjust if days have higher radiation than others.