The back of knees and thighs, and the inner thighs, can be quite sensitive to feathery tickles.
It is rare to find a ticklee that is ticklish enough to get a strong reaction from feathers on the soles of the feet. Those that are that ticklish are referred to as "feather-ticklish" on their feet, generally considered a very promising sign of how sensitive they are!
Despite not invoking laughter on the soles in many ticklees, feathers can be used to explore other areas of the feet. A stiff feather drawn back and forth between the toes or under them may get stronger reactions than when applied to the soles. Having said that, some ticklees love having their feet tickled with feathers, even if it is not extremely ticklish.
These offer a light tickling sensation similar to feathers but with a slightly stiffer point. Try slow stroking over the soles and under the toes for pleasant, light tickling. Look for small brushes like the one pictured.
Quality paintbrushes will tend to be expensive at art stores, but are no more effective for tickling than the brushes sold in bulk packs for a few dollars at discount stores.
When raked up and down the soles of the feet in combination with baby oil, these innocuous household items can become an instrument of tickle torture. Move them straight up and down the foot for one sensation, or turn them side on and closely follow the arch of the foot for another.
Look for the kind pictured to the right, with an convex curve in the bristles that will follow the natural curve of the foot, and small balls on the end of each bristle rather than thin, sharp bristles.
A few ticklees might find this tool quite ticklish on the soles of the feet, although I have observed it to be more effective higher up the body. Try it on the inside or back of the thighs, and behind the knees.
These can be absolutely devastating on the feet when combined with baby oil. In some ticklees, this will be the most ticklish sensation that the tickler can inflict. Dance them across the soles and under the toes, making sure to explore the arches of the feet too, for best effect. Only light pressure is needed, with rapid movement.
Combine this with fingertips, brushes or banjo picks to make the soles of the feet extra ticklish. See a detailed discussion of baby oil here.