Fartlek, a kind of training for runners developed in Sweden in the 1930s, combines continuous and interval running. The activity allows the athlete to run a distance of their choice at whatever speed they wish provided they vary their intensity at regular, fixed intervals.
Free form fartlek (group)
Take turns as leader, deciding pace and distance for next interval; change often, so each runner can lead a couple of times.
Telephone pole fartlek (single or group)
Run hard to first pole, easy to second one; then do two hard, two easy, three hard, three easy; then reverse sequence; do variations of pattern for 20-30 minutes.
Blind fartlek (group)
Each runner gets a number and keeps it secret; as each interval begins, runner with that number takes lead and sets new pace.
Distance fartlek (single)
Choose course — short or long — then decide which segments to run at a sprint, which fast, which slow, which at a jog.