A quiet place to just stare into nature

Boston Common?

Pretty quiet there. Decent parking nearby.

Millenium Park

right outside the Longwood MBTA stop

The Fenway and Riverway sections are walkable from the Prudential Center, though with a bad ankle, taking the Green Line to Longwood would be best. Further up the Muddy River, there's Leverett Pond, Jamaica Pond, the Arboretum, which has both a pond and a separate brook, and Franklin Park with Scarboro Pond.

Magoo’s Wild Wilderness located deep in Magoo’s mangrove swamp. Magoo.

Edit: Magoo’s Wild Wilderness deep in Magoo’s mangrove swamp is a private reserve. Magoo is compelled to address the sickening mid of certain reply commenters. Indeed this is the first time Magoo has ever addressed mean meanies directly. Magoo’s Wild Wilderness is a magical place and in no way intended to mean any place reserved for Ms. Magoo only. Magoo.

(sorry)

https://thetrustees.org/place/worlds-end-hingham/

https://maps.app.goo.gl/ES6MdbipteioyjwR6

The babbling brook north of Wards pond, across the street from the dirt parking lot.
Or the waterfall in the southwestern corner of Wards pond, not far from the paved parking lot.
The rhododendron path along the babbling brook in the arboretum near the South st gate (parking is a bit further away for this, on the newly renamed "Flora" street).
For less people, I recommend finding the fire road in the Blue Hills with the least amount of parked cars.

...is Burlington and Wilmington's Sawmill Brook Conservation area. The eponymous brook has some really lovely and babbling parts as it makes way between the Footbridge and the ruins of the Clapp's Mill dam. There is parking at the Fox Hill Elementary School in Burlington, or at the end of Green Meadow Drive in Wilmington. Also nearby is the much larger Mill Pond Conservation Area, which has miles of trails near and around the Mill Pond Reservoir. It's great hiking. Parking at the Mill Pond Water Treatment Plant on Winter St in Woburn by the Wilmington and Burlington tripoint.

That's my go-to when I want to be outside in nature on short notice.

Some very nice streams with babbling waters are reasonably accessible off of unquity road and the trailside museum although given the current drought they might not be babbling at all.

From 1033 to 1214 on this map

Whether by foot, T or park there, once facing the building from outside, turn left and follow the driveway by the water inlet, proceed down to the river approximately 200 feet, where you will have undisturbed access to the magnificence and wonderment of the Boston amd Cambridge sides of the river, replete with a beautiful sunset. Need extra amplitude, 420 or a choice bottle of wine. Enjoy and don't tell anyone about this spot.

It's a little more than half an hour from downtown, but Great Esker Park in Weymouth is a gem. It's accessible by commuter rail ($10 all weekend pass) and within walking distance from the East Weymouth train stop on the Greenbush line. To enjoy it thoroughly, one needs to be able to walk around for about an hour at a leisurely pace.

Other underappreciated scenic treasures -provided that water waterfront qualifies: All the little peninsulas on the nearby South Shore; Squantum, Houghs Neck, Webb Memorial State Park etc. The adjancent residential neighborhoods are also nice to walk around.

It's the city's largest urban wild, quite close to the Arboretum.

Head towards Rock Pond and enjoy the quiet and the view.

Mount Auburn Cemetery

a truly beautiful, healing sacred space

Google maps says 34 mins, small ponds and a brook plus the Charles

The Neponset Brook reservation has a lovely greenway from Lower Mills to Mattapan, with lovely views & a bridge and a salt marsh and ocean beaches. Standing on the footbridge crossing Neponset River, it’s hard to believe you’re in the City of Boston.