There are a lot of toys that are marketed for budgerigars that are more than suitable for cockatiels. Before giving the toys to your cockatiel, check that they are robust and have no sharp projections inside the toy that could cause injury if exposed.
Simple toys are normally best. Cockatiels love things like mirrors and ladders. They don’t enjoy swings as much as budgerigars do and would prefer another perch to a swing. They might find some stimulation in pushing a ping pong ball around with their beaks. When it gets dirty, simply wipe with some paper toweling.
If you are out a lot during the day, leave a radio turned on for companionship.
The next option is to get a second cockatiel for company. Cockatiels are gentle and sociable birds by nature, and although they may squabble, they rarely get into serious fights, unless you have two males with one female.
Two cockatiels of the same sex will also get on well, unlike larger parrots who can fight violently with each other.