Indoor cats benefit enormously from mental stimulation, and most effective enrichment ideas cost little to nothing.
Rotate toys weekly rather than leaving all toys out permanently — novelty matters more to cat engagement than toy quantity, and a “new” rotated toy gets far more interest than the same toy left out continuously.
Vertical space matters as much as floor space for indoor cats — even simple shelving or a window perch gives access to height that satisfies a strong natural instinct to survey territory from above.
Food puzzle feeding, even improvised with a muffin tin or cut cardboard, taps into natural foraging behavior and slows down fast eaters simultaneously.
A few minutes of interactive wand-toy play daily, mimicking hunting motion, satisfies predatory instinct more effectively than passive toys a cat bats at alone.