I've found that getting out every day (even into the garden) improves my mood and ds's (2). Am pregnant so am aiming to keep this up when the new baby arrives. Our town is about 30mins away, so I do try to and walk in a couple of times a week and go for lunch, swimming, park, coffee shop, baby groups, library as well is a good one.
If I can't be bothered to go into town, our options are walk to the local shop and waste as much time there as possible. Allowing him to help Mummy put things in the basket, woods to pick up leaves, park/field and take a ball and just let him run around.
Even walking up the road holding Mummy's hand will exhaust her, does she have a scooter or a little trike? I got one last summer (cheap Ebay job) or look on freecycle and it was well worth the money.
As boring as it is you could just walk around the residential area counting house numbers and pointing out cars and birds - she will love that.
I normally take him out mid morning every day after I've cleaned up all the crap from breakfast and he is starting to get restless. Get home and give him lunch straight away and then he normally has a good sleep as he is tired from his mid morning adventure....does your dd still have a nap? If so, that might be a good time to work it in or around 3ish for that afternoon lul...
Today is the perfect day to start - we went for a walk about 10am and he is so tired from running in the snow, so you could start today!