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