I think you don't have much time to do more climbing if you need to stay 4 days in Olympus because the distance betewwen Parnasus and Olympus even is not long but the bus take time and also you need to change 2 or 3 buses. At least a full day will be spended on the road.
Rocks of Meteora in very intresting but know excaclty on the road from Delhi to Olympus. There are no any other sighnificant archelogilal site as much Athens, Delhi and Olympus. Althought there are many small but i don't think it is possibe to stop and spend much time travelling with the public transportation.
All the area west of the line Delhi-Olympus is covered with mountains up to 2200 metres altitude and there are many possible hikes but with out well signed routes and mostely not accessed but bus.
In Parnassus mountani you can stay in some of the villages. An other option is to visit Pelion mountain where there are many trekking paths you can see them in http://www.hellaspath.gr/index.php?p=2&m=1&mntid=47
I don't know Parnassus so I can not help you with more detail. If you need info from an other mountain (Olympus etc) let me know.