If you've never been to Greece, Athens would be the obvious go to place (that's where I went for my 2 Segments to get to silver). But that won't be enough.
For the jump to gold I sent to HER. It was nice but a bit too touristy for my liking. Next destinations on the list would be RHO and JTR.
What you could do is LHR-ATH-LHR and on the second day ATH-RHO-ATH or sth similar.
If you really just want the Segments, just pick the cheapest island. But if you've never been to Greece before I think you should set a couple of days aside for it.