I think a cadmium coumpounder would be legit. You could build an underwater room, then spend months in it. Of course for you it might only be weeks, but storing breath then using your stored breath would give you tons of time underwater. Plus at the same time, you would be slowing down time for yourself. You sit underwater for a couple of days, and everyone else sees you disappear for weeks.
It's the perfect way to fake a death. Fall into a lake, and not come up, Everyone will think you've drowned, and then a while later, you just pop up and walk out of the lake like nothing happened.