Just a few ideas..

I'm in a different city, but I assume Sydney has a version of these...

zoo
collingwood children's farm
museum
eltham railway
etc...

say you are doing it for DD, then they can't argue - everyone needs fresh air (museum for wet weather). Take a stroller pram so you walk around a lot (the IL's can sit for a bit in places like this while you keep walking).

Meet them there so there is no backing out at the last minute.

Pack a picnic lunch as a treat for them but make it as healthy as you like.

I have a few memberships to places above, and go to at least one of them every week for an hour or two with a packed lunch. I often say to someone who wants to catch up "let's meet here instead, I've already promised DS". If money is possibly tight, I would suggest a great park that you travel to and like eg your version of Hay's Paddock.

Sometimes we even just take a train to the end of the line for a picnic and then come back. A great day out for the cost of a train ticket.