i rented a bicycle in mindelo and cycled across the island one day. the landscape was very variated and beautiful. just like the people i met along the way. cycling was quite easy as i mostly cycled between the mountains.
you can rent bikes in mindelo from one of the local bike shops.
In the evenings when the sun is not so hot, many young kids and men go own to the beach and play football. Join them if you want. You can always ask if you can play with them, otherwise you can watch them. Buy some drinks and sit there in the sand and watch them.
Cape Verde is a good place to go surfing and to windsurf. Try the kite surf, a sort of surf made with a little parachute. It's also a good place for divings. You can go trekking on the islands of Santo Antao, Fogo, Santiago and Brava. If you are fond of fishing, you can go deep-sea fishing.
Since is such a windy island, it's really good for windsurfing, and there is a stall by the main beach in Sal Rei where where you can rent what you need for several water sports.