Matchbox dinky cars.
For hours, I'd guide various makes and models across the basement floor, through imaginary cities made out of various products. Among the types of cities:
* Low-budget roads made from copy paper, with Dad's books stacked up to make overpasses - pocket books worked best. Nothing says childhood fun like rolling an early 70s stationwagon across an overpass whose road is supported by Who's Afraid of Virginia Woolf? (which I remember trying to read at an early age and mistaking "goddamn" for "Gotham", which left me wondering why Batman wasn't a character in the play).
* Lego, thanks to a gray placemat with a road etched in.
* A plastic road set that I believe was made by Matchbox, complete with a gas station and parking lots.
* Majo-Kit, which I amassed a pile of. Possibly my favourite childhood playset. Besides the basic road pieces, you could buy landscapes ranging from customs booths to tourist farms. Parking meters, park benches, trees, streetside cafes, streetlights, gas pumps...everything you needed to make your own city. Assembly could be as frustrating as putting together IKEA furniture, but it was loads of fun. I suspect Majo-Kit can take the blame for my periodic bouts of SimCity addiction. This site has pictures of a few sets I had.
Of the vehicles pictured here, I had the Mini Ha-Ha and possibly the fire truck. Matchbox made up most of my collection, with only a scattering of Hot Wheels - both lines now owned by Mattel.
Source: Our Fighting Forces #163, January 1976