If it's Mob's first everything will be huge for him, and Reigen's still reserved per "I can't fuck this up by being too much," so it's probably something mundane. Taking the train somewhere they can walk and shop while holding hands, or going to the movies and snuggling in the very back, something simple like that. They're both fine making it a small thing. Reigen buys Mob something small, a keychain or the like, that becomes a treasure for him.

(Site refreshed before I pressed the answer button and I'm hours into fighting Fucking EA right now so I'm digging for what I had in mind before Valentines' ends)