Look for a used chrome-moly steel bike from the 1990s. Many local bike shops or bike co-ops will have them and if you stick with a name brand like Trek, Specialized, Fuji, Giant, or even the better Schwinns you will end up with a bike far better than anything you'll get at a big box store. The hybrids from that era were basically just mountain bikes with 700c wheels. They make great all-rounders.