"Poodpoczywać" to me means you're doing *some* resting (not necessarily full time, possibly implying that, while resting, in general - say, from your day to day chores - it may include some either more active form of resting - like going out to parties for example - or the resting will be mixed with some other activities)
While "odpoczywać" to me implies you're intending to mostly rest in a passive form, ie lay down and rest, less actively, and your main goal is indeed calm resting, with little activity.
Ofcourse, people may twist these for their personal amusement - someone could jokingly say they're going odpoczywać to a shooting range or a skydiving camp, if that is what soothes them.