The five toys that have endured at our house:
- plain wooden blocks (varied shapes/sizes)
- soft balls for throwing indoors
- doll(s) with clothing and blanket
- wooden train set
Also there is a category (for lack of a better word) that my kids call "setups," which is basically a jumbled drawer of small things they can use to set up scenarios - little wooden animals, people figures like Playmobil, "food" they made with modeling wax, dollhouse furniture, small wooden shapes like barrels, horses, etc. This kind of play became important around 3-4 and has continued through at least age 10 (my oldest, at 11-1/2 doesn't play this way anymore). They often use a dollhouse and castle for setups but they could do without - they happily do setups on top of books and make multi-levels with books and blocks.
The soft balls were important when my toddler boys were compelled to throw everything they touched. For awhile we had only soft things out to play with! But they still love to throw stuff and pelt each other. If I was starting over, I would make/collect woolen balls for this purpose (ours are polyester-covered foam splash balls, not as nice but they serve the purpose well).
My girls are totally hyped on dressup, but they could happily manage with nothing in that category but playsilks.
I echo the others who say that art supplies, craft supplies, books, music and story CDs, puzzles, and games are also essentials here but I wouldn't call them "toys."
With the 5 things given, plus "setups" and the "non-toys" I listed, my kids don't need other toys. It took me too many years to figure this out!! All of the things I listed still get frequent use by my younger three (5, 7, 10). My 11 year old seems to be past that stage but he does join in their play once in awhile.