Weight training is a common type of strength training for developing the strength and size of skeletal muscles. It utilizes the force of gravity in the form of weighted bars, dumbbells or weight stacks in order to oppose the force generated by muscle through concentric or eccentric contraction.
Running a set of repeats interspersed with intervals for recovery is a simple way to mix things up. Choose a distance or time period (200 meters or 45 seconds, for example) and run hard. Rest for a set period of time and do it all again (and again).