Seriously! And whatever it is you get them, they can probably already get themselves. So, if it's Sherlock buying something for Mycroft...maybe events that they can go to together (or is that too sentimental)? I loved when in "Cold Blood" they went to a concert together, even if Sherlock said Mycroft was spying and Mycroft said it was entirely coincidental.

Or perhaps Sherlock would allow temporary voluntary access to something Mycroft's been wanting (a-okay cameras in the flat for a week?)? Perhaps the promise that Sherlock will show up to something/do Mycroft a favor that he wouldn't normally do? Guh, you're right, it is hard.