What is the best LEGO set for a kid who just wants to build freely?

Across the reviewers we read, the open-ended LEGO Classic brick boxes (the Medium 10696 and Large 10698) are the consensus choice for free building. Specialist r/lego threads and high-trust retailer reviews repeatedly recommend them as starter or supplement sets because they include a wide brick variety, baseplates, wheels, and a reusable storage tub.