Tennis/running shoes and white cotton socks. If it's a long flight (eg, TPAC or TATL) I'll often remove my shoes (after reaching cruising alt) and place them under the seat or in some other appropriate, out of the way place. I typically travel these long routes in F/J so there's little chance of my going shoeless bothering other pax. I put them back on when going to the lav as you never know what you might find there.