Ireland or Scotland would be good or anything abroad.
Just looking at places we've toured outside of England in the past on the great website that is swindon-town-fc.co.uk.
Most recently we've been to Italy and one training camp in Portugal where we played Rovers behind closed doors.
Most common seems to be Ireland, mainly Republic but one Northern Ireland tour in 99 too. We've also done Finland a bunch of times in the 90s, Cyprus once.
Getting away preseason dried up early 2000s until we didn't go anywhere until the famous Copa Ibiza win of 2006 followed by Austrian tours where Simon Cox scored a wonder goal while we were still in training kits.
We also set up camp in the south of the Netherlands a couple of times in the 90s, playing mostly non league sides there on what looks like a local park. Can't imagine that was the best preparation.
Touring outside of England wasn't very common before the 90s. Outside of a few Scottish tours, we went to Malta in 1986 and did both Germany and France in 1957.