Drive through San Mateo on the way from Las Palmas to the Cumbres and most of what you see in San Mateo is modern. However, the old core of the town, with cobbles, a squat church and lots of olive trees, is a great spot for a coffee or an early lunch.
Unless you arrive at the weekend for the big market there isn't much else to see or do in San Mateo but it is a great base for walking in the surrounding area. The hike up to Cruz de Tejeda is steep slog but you get great views.
San Mateo's market is the biggest in north Gran Canaria and sells everything from fresh fruit and veg to clothes. Look out for excellent local cheese and bread and seasonal fruit that is stright from local farms, Once you've had a wander around the stalls, head out to the square over the road and watch or join in with the dancing.