I feel for you. Since my ex left, he has spent a lot of money on making sure the house is set up for the dcs when they visit - they have everything they could possibly need there (apart from bikes - they go backwards and forwards). Lots of toys, books, new clothes etc. He's doing far more than he ever used to, taking them swimming, to the library, to the park etc.
They do have a lovely time there, and it's hard. On the other hand, I am pleased they are happy there - that would be worse. They are far more moany and whiny for me - he commented the other day how delightful they always are for him. But I do know that's because he has lost their trust - they still love him very much, but they feel they need to be on their best behaviour because he's shown himself to be unreliable, and has let them down, and they feel vulnerable. Whereas I am just boring old Mummy, who hasn't changed, or gone anywhere, or done anything different.
They really really need stability, and you're providing that for them. And they won't forget that, certainly not in the long run.
What about trying to identify some (cheap) things that you can start to do as a family that are new for all of you? We have started having a once a month "Chinese takeaway night". They have a bath, get into their pjs then we drive to the local restaurant, order food, they munch prawn crackers while its cooking then we come home and eat it. We never did anything like that before, and it's a huge treat for them, and doesn't break the bank. Also stories are now in my bed at night, snuggled under the duvet (again, a new thing since their dad left) and we've started taking our tea to the playground on sunny evenings. And sometimes we take our tent to my neighbour's house and camp in her (bigger than mine) garden.