there are airports on both Nangan and Beigan, so some people plan arrival and departure from each island
ex. fly to Nangan then leave from Beigan
Beigan is the 2nd biggest island, but the moutains (hills I would say, they are quite small compared with the ones from Taiwan main island)
here is a guide based on my personal experience to the 5 islands to help you plan your trip,
as I mentioned before, all 5 islands are nice and you won't be dissapointed by any of them,
but when there is limited time you would need to choose which ones to go to and which ones are for maybe next time!