Since Legos are typically sold as kits to build specific vehicles or buildings these days, thrift stores don’t really want to deal with the used kits, because there’s a good chance they’re not complete. There’s always a stray wheel or gizmo that gets accidentally vacuumed up or thrown into the wrong box, and that can be frustrating for the next kid trying to follow the directions. Several organizations have popped up to address this situation, and will cheerfully take your used Legos.
Missouri/Kansas, Kansas City: The Giving Brick has a number of dropoff locations in Kansas City. They clean the Legos up and create new sets to give to kids under the jurisdiction of the foster care system. If you have some free time and an affinity for organizational tasks, they’re always looking for volunteers.
California, Sunnyvale: The Brick Recycler also gives sets to kids within the foster care system, and also to hospitals and overseas. They report they always have more requests for Legos than toys, so send ’em what you got!