Actually, balsa wood is considered to be a hard wood but of course, it wouldn't be very good for shelves.
If it were my choice, I would go for poplar. It is fairly inexpensive and available at the big box store, machines well and with a stain and a hard finish, doesn't look too bad. If looks are more important than cost, I would try mahogany or red wood for weather restance, weight and the looks too.
Just my 2 centovos worth.
Rog