I find that brain stimulation is as important as physical stimulation for our dog when she's been unable to exercise much.
I take her out in the car a lot, if either I am getting out and there is a family member to stay in the car with her, or if I am just picking up/ dropping off one of the DDs. She particularly likes the train station as there are lots of people to look at. I guess this depends on your dog - mine loves the car and is very nosey!
We also do brain games. At the moment I have a badly burned foot and cannot walk her so she is getting short inferior walks with the DDs (I don't want them wandering miles on their own). She was going stir crazy, especially at night 
, so DD sat down with dog and did a long game session yesterday, bless her. Dog was exhausted and slept really well last night!
Her favourite game is for us to get 3 beakers and she has to sit and watch while we put the treat under the beaker. Then she has to go to the correct beaker to retrieve it, or she has to sit and try again. She quickly got too good at this, so now, DD swaps the beakers around whilst the treat is under it. She is a scream to watch - the concentration on her little doggy face! She loves it and wants to just keep playing and playing!
We also try getting a treat and swapping it from hand to hand behind our back. We hold out our hands. She can easily guess the hand it's in - by smell I imagine- so we try making the treat very small and making both hands smell. She still gets it correct, but she does enjoy it.
DD also does language lessons. Our older Sheltie is brilliant at this, but he's 9 and we reckon he understands everything anyway! DD shows an object and repeatedly names it. Then she puts it round the house somewhere and says "fetch the bowl!" (or whatever it is). Our Sheltie dutifully runs off and gets it. Our younger dog is not so good with this, though she excels at the rest compared to our elder one.
We also do agility in the garden, though you won't want to jump with a puppy. But you can get those play tunnels, and put sticks in the grass (my DDs used garden tools) for them to weave in and out of. Our younger dog adores this and it is both physical and mental exercise. Our elder one just runs round the outside and then looks for a treat .
All these things are time intensive, I'm afraid, but will result in a lovely sleeping dog for a couple of hours!