Kids are heading back to school. The amount of daylight is diminishing each evening. Labor Day barbecues are creeping closer -- the (unofficial) end of summer must be near.   Rather than tossing tired flip-flops in a couple of weeks, consider recycling them. Trenton, N.J.-based TerraCycle, which specializes in handling typically tough-to-recycle items, welcomes pairs of the seasonal sandals as part of its Flip-Flop Brigade before they end up in a Dumpster or in the back of the closet. It will retire the worn-out shoes in an environmentally friendly fashion, and those who send in 25 pairs (or 50 individual shoes) will receive an Old Navy coupon.   It's free to participate. For flip-flop recycle instructions go to www.terracycle.com/en-US/oldnavy.   To receive Old Navy coupons, you must ship at least 18 pounds of flip-flops (all brands are accepted). Download a pre-paid shipping label from terracycle.com. You can print it out yourself or request a label from TerraCycle. Ship the box to TerraCycle by affixing the pre-paid postage label and dropping it off at a UPS location.