Surely "they" would think that a supply of chocolate would be a necessary part of colonizing Mars, surely!
I don't know what I would take...
Maybe a paper copy of a history book, as history is always changing and being biased (which is not to say the copy of what ever book I would take could be already extremely so). I would take colourful things - as I look around my office, I have artwork of nature scenes, bright pink post it notes, coloured pens, pencils, crayons, a paper diary for this year.
I guess if all things that you need are planned for, then it comes down to creature comforts - so I might take boldly striped bed socks, or my favourite type of toothbrush - and also how to make a new place feel like home - for which I have no ideas at the moment.
It would be interesting to know what is on their list of things that they think are needed - and in what forms. For example toothpaste - is it really essential (the answer is no, it's more the brushing action that is good for us, than the minty paste), but if it is deemed necessary, then how much toothpaste will they send with them, and how much raw materials with a recipe for the scientist on board would they send?
Gosh, we live in such interesting times!