Go to the moon without any time warp. No cheats, but go kerbal on boosters if you so desire. There are no restrictions in terms of how your rocket is built.
Hardcore mode: come back from the moon.
Legendary mode: To mars and back without time warp.
(If you, for some reason, want to do the mars mission, you may time warp to your burn window.)
Omnipotent mode: come back from mars
I've finally accomplished this challenge in legendary mode: to Mars and back without time-warp!!!
It took really long, it needed practically 6 months in the end. I started on 5 november 2019, with an aggressive injection burn toward Mars to save time:
The ship reached mars on 28 november (during night, I had to put an alarm-clock at 3 am to handle the insertion):
Then the longest part consisted in waiting for the launch window for the return part. It finally came after 3 months, on 24 february:
Finally, my ship reached Earth today after one more month without time-warp:
And finally, here is my ship before landing:
Mission accomplished!