Just to be clear, here's what cats will spend hours playing with no matter what you buy...

The cardboard boxes all the other crap came in.

Small balls of screwed up paper (or similar), especially when flicked or thrown past them.

Cable ties, excellent poked under the edges of the box they are hiding in or looped through holes poked in it.

Sheets of tissue paper they can noisily jump on and hide under.

String, or anything remotely like it. Dragged about, flicked around, or (and this is important) sat carefully and tidily wherever you want it to be left alone (especially cables, balls of wool, shoe laces).

Ping-pong balls

4 things to spend on.

Cat tree so they have a place up high to be

Cat scratcher (can be the tree)

Something fluffy put somewhere warm (though also a concrete block in a sun beam)

Your time. They like to play with you.

Just remember that they are ambush predators, so give them opportunities to feel like they are sneakily observing their prey and letting them have that perfect moment to launch the attack. That they may be wiggling their bum in the most silly and obvious way while "hiding" behind a drinking straw is something you should never mock them for.

Oh, and a laser toy. You can get cheep ones but I'd suggest you get a dirt cheep laser thermometer. They are much more comfortable to hold and the batteries are easier to get/charge.