The Mercado Municipal is San Isidro at its most useful: produce stalls, simple meals, household goods, and the hum of a town that serves the surrounding valley. It is a good place for breakfast, fruit, or a cheap lunch before getting back on the road.
Do not expect a curated food hall. The market is practical and local, which is exactly why it is worth a look if you have been moving through beach towns and park gateways.