GrandmaH
I think the problem is coming up with something original, as you say she already has most toys.
I started to take my DGD to a pantomime at 3 years old and we still go every year. She is 8 now and I asked if she still wants to go 'with nan' and she surprised me by saying she loves to go and never wants to stop. She said 'it's nanny time' and loves it when I take her to the theatre for shows.
It's not a lot but it is something that is a treat for her only as no doubt her brother might not want to go, depending on his age and the toddler is too young. It's not cheap, dependant on the production, I think I have paid about £40 for us to go to Leicester.
Dressing up clothes were a favourite of my DGD too. She had a dressing up box and I bought good clean funny things from charity shops like hats, gloves, boas and jewellery bits, her friends loved 'messing around' too. Kids don't know where they are sourced from it's what fun and imagination they can have with a toy/gift that counts.
Perhaps I am wide off the mark but good luck in your endeavours. Soon be over