When my kids were really young, I would say our best "toy" was the exersaucer. All of my kids LOVED it and it gave me time to get things done.
Just about any doll for DD is a great buy. Wooden blocks, Matchbox cars, tea set, a little riding toy that I don't know the name of, but all the kids love.
We bought a DS for our DS

on his 4th birthday. He just turned 5 last week and still loves it. It was especially great for our trip to MN in May and when we flew to OH at Thanksgiving. He'll forget about it for a little while, but then he remembers it's there and we have to limit his time or he'll literally play from the time he wakes up until he goes to bed.
The Megnadoodle. Let's just say we had to buy 3, so we didn't have any more fights! We got the first one 3 years ago and they still haven't lost interest.
And of course, everything Star Wars, Spiderman or Transformers! And anything dress-up for DD.