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Quite impressive sand desert. You can walk over sand dunes, watch birds and ants (termites?). There is a marketplace close to the ticket office, you can buy water and food. From the ticket office to the desert is a 7.5 km road (single, you won't get lost) to the desert, takes 1.5 hr to walk but you can ask for a "mototaxi".
Of the Cathedral of Santa Cruz.
Zoológico Municipal
Not so big but quite pleasant zoo focused on South American animals. There is a food court and some kiosks inside. Many micros pass by the entrance, e.g. the one to Cotoca.
Go sloth "hunting" with your camera at the plaza of Cotoca and try if you can spot the 6ish sloths in the treetops without the help of the locals. Basically, they are high up in the trees, not moving and looking like a roundish part of the tree, but sometimes you can catch a face or their fingers. You probably will never see them move much, this is just the expectation you have from various documentaries about them, a slowly moving sloth.