I find what helps keep my guys amused is every four years I get a new kitten or two to chase and annoy the crap out of them
Seriously, though, I've just got loads of toys. Most of them are interactive, such as the ones on poles that you wave and they fly through the air trying to get (Aoife is usually far too dignified and disapproving to be seen playing, but she goes so wild and snaky when I start waving the pole toys around that it provides endless entertainment for the other cats
).
I also have a few motorized insect toys that look similar to
these things that they like, but they don't go very far on their own and the cats lose interest unless you constantly set them going.
Once they got over the initial fright of
this thing, some of my guys like it.
Laser pointers are a big hit with my guys, as well. All it takes is the jingle of the chain and they come running for it.
Hanging toys from chairs and doors keeps them occupied on their own, as well as a battery-operated motorized wobbly ball I have (like
this one, only without the fake weasel) that they follow round the apartment for ages.
The three youngest cats love their
crinkly balls and Isobel even plays fetch. She'll bring the ball to me when she wants to play. Isobel loves her crinkly ball, especially once she's dunked it into her water bowl *ewwww*. She even brings them to bed with her and wakes me up throughout the night, playing with it on the bed beside me
Then, of course, there is catnip. I usually have loads of homemade organic catnip pillows lying about (had to put them away when Dougal came to live) and one hit of that stuff and they find ANYTHING AND EVERYTHING amusing
I have GOT to get this
'Thing in a Bag' - that looks like it should be a blast for them!
I would love to have some cat trees for my guys but they're so pricey here :-( I've got some reasonably funky scratching posts, though, that they can climb over.
FAQ 
It's hard to keep him going for some meaningful exercise.
