"I can take something for you."
Translation:Mogę coś wziąć dla ciebie.
There's none. Although it's Genitive, and "cię" is also a Genitive option, it will not work here at all. I don't know how to explain why, unfortunately.
You can change the construction to Dative and it will be almost exactly what you wrote: "Mogę wziąć ci coś", "Mogę wziąć tobie coś".
The way I was taught was that when it follows a preposition, ciebie is used instead of cię. So it's: "Szukam cię" and "Nie lubię cię" but "Idę do ciebie" and "Jestem obok ciebie" and "To jest dla ciebie", for example. There may be other reasons ciebie is used instead of cię in the Genitive, but certainly following a preposition is a major one. This exact same thing happens in Accusative as well.
I've heard people say both - "coś wziąć" and "wziąć coś" often enough that it's hard to tell which is correct, though in this particular sentence "coś wziąć" does sound more natural to me. If you'd switch them in this sentence when speaking, probably no one would notice.
It may not be the case in other sentences, though. For example: "Jeśli to ci się nie podoba, mogę wziąć coś innego" (If you don't like this, I can take something else) - in this case you could not switch "wziąć" with "coś", that would be definitely incorrect.