Da Bird, as far as toys go.
We have the Go Fishing wand toy - never bought replacement fish for it. We just tie the tails of little furry mice onto (our kitties only like the kind that make a rattley noise) the end of the string. They love this even more!
Other than da bird, we "make" most of our wand toys these days. The handles of toys that got destroyed work great. We used to just buy dowel rods at Home Depot, and that stuff used for laniard or leather shoelaces, and use duct tape to tape whatever string-type thing to the handle..... and then just tie stuff on the end. Milk rings, the furry mice, a wad of paper, stringy packing material..... so long as it's moving, and it "feels nice" when they catch it - or it makes a good noise - our kitties love it.
But the best investment we made in our kitties (apart from their health!
) are the cat trees. We were lucky in the sense that a non-chain pet store was going out of business. They had expensive cat trees on sale at FABULOUS prices (or we would not have been able to afford them). We bought two pieces when we were still in the RV. We moved into a house before the store was closed, and we went back and bought a whole series of the same style in every shape they had - then ended up going back to get whatever was left. They have turned out to be SUCH high quality - the original piece still looks fabulous with almost no wear and tear, and it has been - almost six years now?
The point of that whole ramble was.... if you ever decide to invest in cat furniture, save up the extra money to buy high quality. There are condos out there that look so much fun, but they screw together, cost $200 - $500 and fall apart in 2 years (we purchased one of those too).
The other thing I'd mention about trees, if it's something you ever want to consider, is put two or three of them next to each other, in front of a window. Our cats get into FABULOUS games of chase and LOVE being able to use all of the different levels and heights - jump from top to window sill, bounce over to the far tree - it is truly like a cat jungle gym or something.