4-track Loop
A 4-track reel-to-reel can be used to make a tape loop. You need to be moderately handy with marking, cutting, splicing and setting up something for the loop mechanically to go around, but it's not beyond most people.
Using a 4-track cassette deck strikes me as a less rewarding exercise. You can buy "endless loop" cassettes (answering machines use them, among other things), but you won't have much flexibility in the length of your loop. The only way to vary it is with the pitch control, and you can't set or change the length after recording. If you're clever and handy with fine work, you might want to try cutting tape and making a cassette loop. I don't.