Lode Mill and Quy Fen
Lode water mill is part of Anglesey Abbey - a National Trust property with delightful gardens - but unfortunately these are not open to dogs. You can, however, approach the mill without going through the Abbey grounds, and then walk along the other side of the river and around the open fields which adjoin it.
There are a mixture of paths: along the river, through woods, and alongside fields. It's easy walking in the summer, but can be quite muddy in the winter.
The following circular route is just one option - this is a little under 4 miles and takes about an hour and a half. It conforms approximately to this track I've uploaded to EveryTrail.
If coming from Cambridge or the A14, take the B1102 through Stow-cum-Quy. (Quy rhymes with 'why', in case you're wondering!) Park in Anglesey Abbey car park and follow the path which goes through the back of the car park, until you reach the mill.
A path around the right-hand side of the mill and over a little bridge takes you round to the river. (I, instead, struck out along the foot path which is straight ahead of you as you cross the little bridge; it take you along the left side of a field, into a pleasing beech wood, and by turning left in the wood you get back to the river.)
Follow the river away from the mill through the wooded area and it will eventually emerge to meander between raised banks, with fields on either side. Eventually, this will meet the (relatively quiet) Station Road. You need to turn right and then turn right again onto a path which will take you back, parallel to the river but in the opposite direction. If it's not too wet you can avoid the road section by crossing the last field to a kissing-gate which lets you onto the same path.
The path goes straight for a while and then bends to the left, heading NW away from the river, becoming a road-like bridleway between trees. When it bends to the left, turn 90 degrees to the right along another wide path between trees which eventually comes to a gate into an open meadow. There's a 3-way signpost here - go through the gate and turn right in the Lode direction, along the right-hand side of the meadow and through a couple of gates. Just around a corner to the right there's another gate and a small bridge - cross this and go straight ahead through a grassy field back towards the river, though before you get there the field bends around to the left and joins another wide track.
Shortly after a dilapidated building on the right there's a turn off into a narrower path to the right, which ends at a cinder track. Turn left - you're now going roughly parallel to the river again - until you get to the end of a wood - the wood you may have gone through at the start of the walk if you took my detour. You can turn right and go through this to the river, or alternatively go a little beyond it and round to the right, then cross the ditch to an open grassy stretch which will take you back to the river between two wooded areas.
Turn left and you'll return to the mill.