My Mom gets full credit for this bright idea. Keep an old sheet or tablecloth with the legos. When it’s time to play, spread the sheet, and play on the sheet. When it’s time to clean up, pick up the corners, and dump it back in the box. Easy breezy, one, two, three-sy.
I must say, while this worked extraordinarily well with me and my sisters growing up… not so much with my boys. The big one has the general idea, but the Legos almost never stay completely on the blanket. But! It’s so much easier to pick up the small amount that wanders off, and toss them on the blanket before dumping the whole mess into the bucket. And my son loves the dumping part.