Well, a lot of folks go for walks in the countryside or go to lunch - or both. Hubby works in a restaurant, he says that they're often busy on Sunday afternoons.
When I was spending less time other days with m-i-l, we frequently went to lunch or for coffee (and dessert) on Sunday afternoons. (If we were still doing that now, I'd be seeing her almost as much as I see my hubby )
Some go shopping in the shops in major train stations, others go to the lake, some go to church, some stay at home, many make excursions by car or by railways or by tram, many go to museums.
Sundays are horribly boring here for me too. When I first moved here, the quiet and everything closed got to me. So I started to organize things as the weeks went by. Now it is waking up a bit late, nice brunch followed by tv shows, having tea, doing the house chores, I do not care if they say you can't clean on a Sunday here, I go on with that. Read a book. Go out for a walk usually in the forest nearby, followed by a coffee at a restaurant.
Down to Migros at any of the open stations to grab stuff for dinner. Home, relax a bit, cook a nice meal, dinner and reserve seats for a movie in the cinema. Sunday flies by and I have a good sleep at the end of the day.