I don't know where the Heart of Greece tour stops but a couple of suggestions:
In Delphi I stayed at the Acropole - http://www.delphi.com.gr/acropole/index.html - which was very comfortable and had great views.
In Athens it's worth looking at the Tempi - http://www.travelling.gr/tempihotel/ - or the Attalos - http://www.attalos.gr/more.htm.
In Gythion we stayed at the Pantheon - http://www.gto.gr/eng/accomm/hotel/detail.php?id=5969 - which was fine, but I'm not convinced Gythion is actually worth a visit. It's a nice little provincial town but not much there.
However, I would add Sparta to the list if you are interested in sites. Mystra, which is about 20 minutes by bus out of Sparta, is really beautiful. The home of the last Byzantine emperors once Constantinople had fallen.
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