Setting aside the issue of how the Whites were supposed to plausibly afford a time-consuming hobby with no apparent finished products, Skyler is a controlling shrew and would have dug into anything he got up to. She's a Lambert woman, so no matter what excuse or hobby he made up, she would have found a way to make it about her. Hell, the cancer would have been a reasonable cover for odd behavior with most people, but Skyler stages an intervention over some minor inattentiveness on his part, obviously giving her version of Walt's transgressions to Hank & Marie to get them to play along. Once they hear his excuse, they immediately switch to his side, as it is plain that his version is pretty damn reasonable, so she storms out.

When Skyler quizzes Marie on the effects of smoking pot, it is plain that she is only looking for answers that confirm her own suspicions, and she tries to go around Walt by scaring off Jesse when it is plain that he's not going to concede the moral high ground on this issue. She takes one slip of the tongue and backtracks it to unravel his whole story. She turns passive-aggressive and starts leaving the house for hours at a time for no reason to punish Walt for not giving her up-to-the-minute status reports. The stay-at-home mother of a single low-maintenance teenage boy demands to know where in Walt's busy schedule he is going to fit in the chore of painting a room for a newborn baby that won't care, or at least won't object to the color scheme, and makes a big deal about it when she is finally forced to get off her ass and do it herself (my mother raised up to nine kids ranging in age from 2 to 16 at once, with no domestic help and a sister who lived out of state and did her own damn house painting * or wallpapering because she appreciated a husband who worked multiple jobs; and she hit the workforce again when her youngest was old enough for other people to babysit, much less possessed of a degree of self-sufficiency comparable to Walt Jr, and she'd have never quit for something as frivolous as disliking the fumes in a factory adjoining the office where she crunched numbers).

The point is, Skyler would never be satisfied with a good excuse. She demands accounting of his time, and that he report to her even when she has nothing else going on, or do superfluous things for her. I can count on zero hands the number of times my mother dragged my father along to buy clothes for a single kid (or kids, plural - that's a one-person job, and if Dad didn't have work at the moment, he had damn well earned his R & R), but even such minor tasks must needs involve the entire White family. If Walt came up with any sort of hobby, Skyler would have been all up in it, setting up playdates with other hobbyists, checking the internet to verify conventions or trade shows Walt said he was going to and foisting Junior or Hank on him for company.

The woman could not even let him run his criminal enterprise in peace until she saw on the news that a drug lord who frightened the DEA was killed when he crossed Walt, and was finally cowed into spiteful submission.

Also, she was fairly indiffernt to Walt's interests when the show started, which might have led him to assume he could get away without checking in on her (note the scene in bed in one of the first episodes where Walt is telling her about his plans to visit some sort of science museam exhibit and Skyler could not honestly give less of a shit).

So on the one hand, the most plausible excuse in the world would not have held off Skyler's inquisition (Walt is probably the one husband who DOES expect the Spanish Inquisition), and on the other hand, she doesn't care what he does, so why bother going through the rigamarole?

And too, the show is about the power and freedom Walt finds in "breaking bad" and blowing Skyler with flimsy excuses is probably part of the fun; like a macro version of the "Get down off my ass" speech.