A surprise awaits near Collinsville, 8 miles east of East St Louis: classified as a Unesco World Heritage Site, with the likes of Stonehenge and the Egyptian pyramids, is Cahokia Mounds State Historic Site. Cahokia protects the remnants of North America's largest prehistoric city (20,000 people, with suburbs), dating from AD 1200.
While the 65 earthen mounds, including the enormous Monk's Mound (from which you can see the Gateway Arch and the St Louis skyline on a clear day), are not overwhelmingly impressive in themselves, the whole site is worth seeing. If you're approaching from the north, take exit 24 off I-255 S; if you're approaching from St Louis, take exit 6 off I-55/70.