Maybe you can cut some of the time you spend with your mother, and find new hobbies and interests for yourself? Since perhaps your mother is content with how your time with each other is currently spent, and may not be interested in finding new things to try out? Of course, I don't know your mum so I may be wrong. But it's also no guarantee that both of you would enjoy the new activities you will try out. If it so happens you end up enjoying some more than your mother, then you've found hobbies for yourself.

As for things that you could try that don't require much financially. Maybe you can cook new recipes with your mum? Or she can teach you to cook things that you don't know how to cook?

Expanding your physical activity together beyond walking maybe? it doesn't have to be anything too exerting. There's quite a few light exercises that you could find by just a google search. Or it could just be a hike, jog or swim. Usually, whenever I go walking I make an effort to find a place to walk where I haven't been before.

Another idea is, what does she do in her own time she doesn't with you and vice versa? If it's anything that can be done with more than one person, perhaps you can introduce each other to your individual hobbies and interests.

all i've got for now