do like I do for the relatives.... Buy them a bottle of some cheap, plastic bottle booze and tell them its great to see them, but they need to get the fcuk out of my house....
You could get him some new brake lines. The Fren Tubos are super sweet and priced pretty well, not to mention they make a nice little difference in the performance and safety of the bike.
Stabilizer? I would say that was the best addition I ever made to my r6 outside of tires and brake lines. Can be had reasonably priced for less than $500. I loved my scotts, less excited about my gpr.
520 conversion and -1/+2 gearing. Can be had for sub $200 all day and is the most noticeable gain in performance for the money you can do. More than exhaust.
Rear shock that doesnt suck. This is more of a track mod, but if you want to get the most out of the bike at the track, the stocker has to go.
Case covers and sliders/spools. If your brother in law sucks at riding, this is pretty much a solid mod. Compliment of spools/sliders can be done for sub $200.
If you are baller status, get the guys some rims.
Im a function over form guy, so all those silly mods like levers, fender elimators, HIDs, and so on are wasted on me because they do zero for me.