LOL my DS is a similar age (18 months) but I have to admit his absolute favourite toys are his cars, lol... oh and he loves balls too. However all kids are different, and will like different things.
We are also getting him a sandpit for Christmas, and a big Tonka truck which he can play in there with.

Do you think your DS might like some sort of musical instruments? We bought our DS a $25 kit from toyworld which had a tamborine, bells, maracas (shakers), triangle, mini cymbals, etc. He also loves having a play with my glockenspiel (xylophone) which I had when I was learning percussion as a child. I think you can get cheap xylophones at toyworld too...

Have you thought of a swing maybe? We bought a $15 baby swing (suitable from 1-4 yrs I think) from Kmart(Big W has them too) and it just attached to the rafters on our patio... very easy and he loves it!

We dont have a table & chair set for him yet, but are definately thinking of getting one.