Might depend on the device, but for most current devices you don't need any extra gear for indoor running to work, as pace info is derived from built-in accelerometers. To achieve good accuracy the watch needs to calibrate itself during several outdoor gps-enabled runs. To achieve even better accuracy, you'd need an external footpod. Indoor biking needs at least an external speed sensor or any compatible external source of speed, otherwise the watch won't be able to track anything except time.
Indoor cycling doesn't really need any extra sensors, but without them all you'll get is time and HR. I haven't bothered with a speed sensor but I do use a cadence sensor to log the turnover and just enter a distance manually on the mobile app after the ride is saved.
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