Sooner or later, if you get serious about bicycling for exercise, you will want a road bike. If you like a women's style, the vintage mixtes are tough to beat. I sold a Panasonic Sport LX in January for $135; alloy rims, three piece crank, cromoly frame. I think any of the Japanese mixtes from the 80s would be nice, and can be found in the $100 to $150 range.