The town we live in--Winchester--is a painfully obvious choice given your criteria. It's six miles North of Boston (or a twelve-minute train ride), a beautiful town, and boasts the Middlesex Fells--a 2,000 acre park and nature preserve with hiking, mountain biking, etc. It is also the most dog-friendly town you could ever hope to find.
Winchester is bordered by both 93 and 95, so access to Andover (reverse commuting) would be a snap. Condos are relatively affordable--houses much less so. It's a "family" town--and nobody would move there for the nightlife--but it's a ridiculously easy drive into Boston for dinner, shows, etc (it usually takes us under 10 minutes). It also offers that walkable downtown of which you speak with one exceptionally good restaurant (Catch) and several decent ones.
Most people will tell you that Winchester is great--if you have a family (it has the best school system in the state--which is a nice safeguard aginst depreciation). I would argue just the opposite. We recently had our first child and are now looking to move out further so we can get a bigger house/yardfor the money. Most of the near suburbs of Boston are a little dense for small children.
Affordability isn't really the Boston area's strong suit (and Winchester is certainly no exception)--but would you ever consider a condo? Even the nicer towns are manageable if you don't need a big house to accommodate a family.
40's and child-free in the Andover area? No thanks. Lexington and Arlington are nice towns, but nowhere near as accessible to either Andover or Boston. Davis Square, Somerville and Charlestown both have a lot of cultural offerings, but are a little too young/busy/dirty/noisy for most 40-somethings.
Feel free to drop me a line if you need some other suggestions--each town around here definitely has it's own distinct "flavor" and I would hate for you guys to choose the wrong one. I'm at email@example.com.
A great way to get to know a community is through its newspaper--here is a link to Andover's hometown newspaper: http://www.andovertownsman.com/
There is public transportation to Boston via the T http://(www.mbta.com) and getting to the seacoast is an easy trip as well. Overall, Andover is one of the most sought-after communities on the north shore, and while prices are somewhat higher than some of the nearby cities and towns to the south (especially if further from, and without public transport into Boston) Andover remains a popular destination.
There is everything from upscale condo developments to stately 200 year old homes, and everything in between, including rentals if that's what you are interested in.
If I can help with your search, please feel free to give me a call or send an email.
He is a google map with a bunch of dog friendly places:
Another interesting place to consider would be jamaica plain near the Pond. They have very pretty trails plus it is about 10/15 minutes to downtown.
I grew up in Philadelphia, my entire family still resides there, wonderful place. You will enjoy Massachusetts a lot ... gorgeous new england feel plus close to the Cape which is amazing all year round, and close to NH and ME which offers awesome summer activities as well as winter activities.
Wakefield, MA could be a good fit with a lake to walk around, a commuter rail to Boston and 95 and 93 just down the street!
I am a Realtor in Essex County and would be happy to assist you in any way. The key is going to be what price range your budget falls in. Andover is a very nice town but is on the higher side of the pricing scale. There are many communities with more of a city feel and still on the commuter line to Boston. If you go to my website you can use the community research page to get a better feel for price ranges and will have access to the Official City/Town websites for all communities in Essex County including Andover. If I can answer any other questions you can feel free to contact me through my Trulia profile or my website. Best of Luck!