Tennis racquets and tennis shoes.
RACQUETS ... I have one racquet I use mostly when I travel to low altitude; it's a little heavier than my others, thus puts a little more oomph behind the ball. (Playing tennis in higher altitudes is tough, because the ball tends to fly faster and it's easier to hit it out. It's why you don't see professional tennis tournaments in Colorado :( )
Then, for my regular racquets, I carry a couple that are strung at slightly different tensions, or that might have slightly different strings in them, depending on if I want more spin or more power or more control on a certain day.
SHOES ... I play on both clay and hard courts. So I have clay court shoes, which are a little softer and lighter and have a tread on the sole that works better on the dirt. THen I have hard court shoes, which are heavier and have a little more cushion and durability ... and within those, I have practice shoes and match shoes. (Match shoes are lighter weight, for speed.)
I don't even tell my tennis buddies this, because they'll think I'm weird. But most people don't understand the "need" for more than one pair of skis, either, so I know I'm in good company.