Walking is out for me due to disability. I can't drive or use buses and unless someone takes me out, I'm confined to my home and often get 'cabin fever' 
I sing sometimes - alone and with the doors shut so the neighbours don't complain.
I go to an elders group, but not if I'm feeling miserable; swim therapy is good (there's a shuttle to the pool but it has to be booked 24 hours ahead) but again, if I am very down, I have no impetus to do it.
I live near the bush so birds are lovely but if I'm really miz, none of this can cheer me.
There's no one I can ring to come for coffee, so I just have to wait for it to pass - and it usually does. 
I'm not really the 'sad sack' this makes me sound, it's just that due to a visual handicap, I cannot do any of the things which used to cheer me up, such as getting out of the house, going to the beach, taking a picnic to the park etc.