Here is the basic list of cat enrichment ideas for indoor cats which behavioral specialists repeat time after time.
Hunt, catch, eat (5 min)
Play with a toy on a string which simulates hunting prey, hiding behind furniture, darting and pausing. Then let your cat catch the toy and feed it with something. The "hunt-catch-eat" scheme suggested by Jackson Galaxy is a full cycle which is the best way to keep your restless cat busy.
Food puzzles (DIY)
As Leticia Dantas, Mikel Delgado and their colleagues revealed in 2016 in the Journal of Feline Medicine and Surgery, there is a correlation between using food puzzles and weight loss, lower levels of anxiety and fewer attention-seeking behaviors in many cats. First of all, you have to get an easy puzzle so that your cat could succeed easily, then make it more challenging.
Window seat with a view (Free)
Either clear the windowsill or install a window perch for your cat. Install a bird feeder outside if possible. "Cat TV" costs nothing and entertains a cat for many hours.
Scent trails (Free)
Apply a bit of catnip or silvervine to the trail of objects or bring in some natural objects which have different scents (pine cone, leaf, cardboard box from the shop). As Sebastiaan Bol and his colleagues discovered in 2017, about 80% of cats react to silvervine, which is more than to catnip (66%). So you may try silvervine if your cat doesn't react to catnip.
Clicker training (5 min)
It is quite possible to train your cat to sit, give a high five, respond to calling or get into a carrier. Training is a kind of brain exercise and cats which are trained to perform certain tricks are more confident.