Hi,
My DS is 26 months,I have a few more things to add to the list of possible toys.
I haven't seen playdoh mentioned, Playdoh is one of those lasting and inexpensive toys for us, with nearly daily usage. DS can't make intricate shapes but makes "pancakes", "beans" etc which he feeds to his dolls, and together we make more advanced stuff.
A older-child or adult-level photographic guide with an index, to whatever interest the child has (in our case birds). We also have a children's dictionary that he likes to look at.
Also this may seem a bit premature but we got him a CD player for kids (Sony model ZS-E5) for his 2 year birthday and its not too hard to use independently, he loves it, is gentle with it and has a few CDs of children's songs he plays.
We have tinker toys (Tinker Toys are sticks and connectors to create structures, one can create large things quickly which is nice compared to lego for example), DS enjoys them but currently needs an adult to help make anything very big (can't press hard enough to make the pieces stay together well), so finds it a little frustrating still.
Just recently got a dollhouse and that has been excellent.
I second the train table idea, we got ours used on craigslist and it came with a huge bin of track and structures. If one doesn't want to do the consumer thing and buy a actual train table one can just dedicate a coffee table height table to be a play table, draw on some roads with washable marker (or with masking type tape), add some toy vehicles, make broccoli trees, a house from a box, etc.
I do find our house cluttered and sometimes think to get rid of a lot of unused items, but like you I find if I put it away and bring it out again it is novel and interesting for a rainy day.
Oh, one last thing, DS enjoys matching games so we have a dinosaur "matching game" -- these types of games are usually rated 3 and up for no good reason. Sometimes called "Memory" and available in a range for a range of interests. Ours gets probably every 2nd day use for the last few months. The GoodNight Moon game is similar.
Polly