Feathers are the most classic, old-school tickling tool. You can find real and fake feathers of various style cheaply online. Large, soft feathers like ostrich and peacock feathers cover more skin area and are more effective when softly dragged or brushed over the chest and sides.
Stiff, smaller feathers generally are more effective at the tip, which you can flick over tickling zones. Feathers rarely elicit the strongest reactions, even when you brush them over feet, the backs of knees, inner thighs, and other hyper-tickling body areas. Brushing a stiff, smaller feather over or under the toes (or running one back and forth between them) might not put your partner in fits of laughter, but it’ll get them good and warmed up for what’s to come.