Menotomy Rocks is a great place to take your pup. Pro's: pond that pups can cool off in, off leash hours in the morning, nice woods to walk around in. Con's: pond gets a little funky towards the end of the summer and it's a beautiful spot but not huge. Maybe takes 45 mins to stroll the whole park. Still your pup will love it. There are also community bulletin boards, trash cans, and a playground for the little humans. Street parking on Jason St. in Arlington.
Thorndike Dog Park- Arlington's Dog Group, A-Dog, fought long and hard for this beautiful park. Great location on the bike path. Pro's: Fenced in dog park with big tree in the middle so there is shade. Fresh water available in warmer months and shoveled gates in the colder ones. Dog owners galore texting on phones and chatting with their neighbors. Lots of trash cans and bags for picking up "presents". Con's: sometimes there is a crazy crowd like any dog park. My pup doesn't love the pebbled substrate.
Mystic River- Follow the path on the Arlington or Medford side about 30 mins from River St to Pleasant St and back. No trash cans. But lots of places for your pup to sneak in a swim in the river. Not far from busy roads so this only applies to pups that are spot on with recall.
Alewife Greenway Bike Path- Across from the Thorndike Dog Park, there's a board walk leading to a sandy path that follows Rt 16 towards Medford. Heads to Dolby Field and Mystic Valley Parkway. Not as highly used as the Minute Man Bike Path, more greenery, but pups still have to be comfortable with bikes, runners and other dogs passing in close proximity. Not many trash cans.
The Res- you can reach the res trail from the Minute Man bike path and Hurd Park or there is parking on Lowell St. Dogs are allowed off leash on the Lexington side but not allowed in the water. Arlington residents can swim in a section of the res during the summer so parking can be tight when they are open.

Everyone has neighborhood place to take their pup for a stroll. What's your favorite park?