Visit a few bike shops and you should be able to find a good bike for under $400..maybe under $300 if you can find a used model/tradein. Skip the disc brakes on cheaper bikes, they are nothing but trouble and added weight.
I used my mountain bike for just what you describe for years. I did ride a lot of off-road, but that was before I really got into mountain biking, so it wasnt much more that leisure riding, not real 'mountain biking' down wicked paths, etc. I rode on some Bontrager Comfort B 26"x1.95 tires as a compromise; they have a smoothe center tread, and some knobs on the shoulders to help you corner in the dirt. Eventually I bought a complete second set of wheels and put some 1.5" slicks on them for road, and put some 'real' mountain bike knobbies on the other set of wheels so I could switch easily.