Dump blocks on the floor. Put a few small animals or figurines nearby.
Use four blocks to make a square around one animal. "This one lives here." Step away.
Within a minute most 3+ kids start fencing every animal. Then making bigger pens. Then "the lion escapes" narratives. Heuristic play in the loose-parts sense — Goldschmied would be proud.
"Now build a fence that all the animals fit in." "What about a fence with a gate?" "What lives in the smallest pen?"