For the first 6 days I would use my gagamba powers and fabulous mustache for things you would expect like getting around easier, climbing surfaces, and having a fabulous mustache. On the last day, I would do a bunch of super-illegal things like theft, extortion, and assassinations. Then I'd remove my mustache and mail it to my normal self's house so when I switch back I can still have a fabulous mustache.
*edit On the plus side, seeing Mephisto running around in my smol adorable body would be so fucking cute. Hopefully he'd be able to get me into madami shape since he's a perfectionist... ...but then again, he's skinny as hell so I dunno if he cares about body shape. I would hold him and cuddle him and snuggle him. Because he'd be me. And I'm smol. And Mephisto is tol. He'd probably be like a cute little tsundere in my body tbh.