What I'd consider a must: a Cat tree or two and scratching posts everywhere. Beds of the cave variety and up high variety (better yet if it integrates with the tree). If they're a friendly sort, figure a bed near where you spend time or prepare to lose an armrest. Not all cats like sitting in laps (especially if it's not cold) but might still want to be nearby.

Different cats have different preferences. Sticks with a dangly thing on a string for interactive play seem super popular but they have to be in the mood. Flingable toys and cat nip infused toys especially vary a lot with the cat. Consider if things will get accidentally vacuumed when considering flingable toys. There's some "robotic" toys that spin around (either with dangly bits or laser bits) that they might like but they can also be very hit or miss, aren't as cheap, and require battery maintenance.

Try to do what you can to give them a nice "cat haven" that's out of the way and quasi-outside but without the risks and bird killing from actually being outside. At the least a window perch but access to an enclosed porch or such is ideal if you have it.