Lovely idea for a thread OP - so sorry to hear what you went through
I think you are dead right when you say it's the little things that can make kids feel happy. If you are thinking of that you are a great mum.
I think mine would be things people have already mentioned, like warming clothes on the radiator etc. We used to have "treat" teas sometimes during the week, which would be a quick tea like fish fingers etc, but we were allowed to eat it in the living room and watch TV - always an old b&w film like Sherlock Holmes or Tarzan 
My mum used to buy me books sometimes as a surprise, even if I hadn't asked for them, I used to love that. She used to write a little message inside the cover - have still got most of them.
She used to do treasure hunts for us as well - except we called them "mysteries" as she used to leave puzzles/clues for us to find.
My dad always used to play games with us (board games, but blind man's buff etc as well) when he got in from work. Also for breakfast, he used to cut our toast into shapes like a house or a star.
They're both still lovely and kind and caring now - eg if I go and stay with them, they put slippers out for me in the spare room and make me tea and toast etc!
Also, this is not a parent but something I always remember: we had a lovely childminder who sometimes used to put a sweet in our coat pocket - she did it secretly, so we'd find it on our walk home from her house.