Anything in any park is the BEST in my 18 month DD's eyes. Also anything that involves lots of other kids as she is very social so playgrounds, library story time, kiddie music groups. She's also mad about animals so the aquarium and zoo are favourites.
we live out of Melbourne and rarelyuse PT, so taking a train, tram or bus is very exciting.
love the zoo, but try to go on non school holiday days.
beach
sjate park
watch street performers
Myer Christmas Windows
go for a drive to see Christmas lights
painting
cooking
sometimes it is better to not to try fit too much in one day, or one week. enjoy the moment. for example, we could go to the zoo and spend all day and see every animal and end up with cranky exhausted kids and parents OR we can pick 3 animals, or one loop, and go at kid's pace and all have a more enjoyable day.
if you can, spend time with one child at a time, and make it their special day.
Anything outside is popular here - nature walks, collecting things (which you can then come home and make a collage of, paint, sort into boxes, use in playdough etc), park play, hide and seek outside, picnic lunch.
At home, cooking is fun. DS also loves ooblek (google it!).
A trip to the library to play with their puzzles, games etc
Museum, zoo etc - and yes, the bus/train trip is part of the day out
Anything with water will be popular in Summer...water play, coloured ice cubes, water parks, beach....
Morning or afternoon tea in a cafe (not sure how an 18 month old will go with that one)
Bookmarks