you want to do mostly around town riding with the occasional mellow dirt trail/ very mellow mountain biking? go with a singlespeed road bike, there are a lot of burly more "freestyle" oriented singlespeed/fixed frames out there that work very well for cruising around town, and having the strength and tire size to hop curbs, stairs, beat on, ride smoother mtn bike trails etc.. they are made more for the fixed freestyle crowd (which is pretty lame IMO) but they happen to make awesome all around bikes that lean a little more towards around town riding. check out the milwaukee bruiser and volume bikes for a start. get some burly rims, bigish tires, front and rear brakes, and a couple different freewheels, one for around town, and one for trail riding..
check out the surly cross check too, for pretty much the most versitile, in the middle bike i can think of..
salsa vaya is similar too.
if you want something mtn bike/trail oriented, but still used a lot for around town riding. get a rigid 29er. and 2 sets of tires. check out the surly karate monkey, salsa bikes, vassago.
if you want more mountain bike oriented, and will get you around town pretty well. check out hardtail 26 and 29ers. 29ers will be a lil faster in general, 26 more fun and easier to throw around, handle better. dont go crazy with fork travel. theres tons of options out there.
if you want very mountain bike oriented, will get you around town, just not quickly. look around for aggressive hardtail's id say go 26. but there are some 29ers out there that are more aggressive and supposidly pretty fun. transition trans-am, on-one 456, (456 summer season for extra aggressive)(also very good deal price wise) check out ragley bikes. just look around till u find something u like.
really, anything hardtail will get you around town well. but if most of your time is going to be spent biking on road, going fast. its definitely very nice to have something more road oriented. but if you think you are going to do a lot of mountain biking. something with any road oriented design in it is going to hold you back. and going for a hardtail with a suspension fork will be a lot more fun, and still get you around town fine. just not near as efficiently.
personally if i could only have one bike, for everything. it would be a rigid singlespeed 29er. like the karate monkey. because i mountain bike a lot and put that above riding around town even though i probably ride around town more.