West Rim out and back is easily doable in a day, the part up on the mesa is pretty flat. If you care to pay, you could book the Zion Adventure Co shuttle up there, hike down to the valley, then take the park shuttle back to Springdale. With an early start you could tag Angels Landing at the end (which is nicer late when there are fewer people, pack a picnic and eat dinner up there).
By canyon standards the Subway is dead easy, but that hasn't stopped a remarkable number of people from hurting and killing themselves in it, mostly in really dumb ways. If you're a sure footed climber with a bit of multi pitch experience, the Subway is well within your grasp. If you've never rapped before, it probably still is, but you should go practice a bunch first.
Orderville might actually be a good shakedown for the Subway. There are 2-3 short downclimbs/handlines/raps which would give you a chance to practice. The hike of Orderville is quite a bit longer and harder than the Subway, but the little raps are easier (mainly because they don't involve running water.