I have a chain spinner ring that I got years ago (like this one) that I wear almost all the time. Some people do well with fidget cubes, spinners, tangle toys; some people just keep a small piece of fabric with an uneven hem to run under their fingernails. It can be a very personal thing. If you have a Target near you, they tend to have a lot of good options for decent prices. Find something that strikes your fancy that you can mindlessly fiddle with one-handed.