Latha, for Meteora, I had a good experience with Visit Meteora Travel, a few years ago. My 2-day experience included round-trip train, nice hotel with breakfast, guided sunset tour of some monasteries, then guided morning tour of other monasteries, plus some free time to explore Kalambaka, with dinner then lunch on my own. They have different length experiences, some include hiking tours among the monasteries. All went smoothly and on schedule. The train ride between Athens and Kalambaka was 5.5 to 6.0 hours each way, quite tedious I found, so I cannot imagine doing this as a day trip. One would spend so much time on the train, with very little time at the monasteries.
I also took the excellent RS Greece Tour, and followed with 3 nights on Santorini, the 3 nights on Naxos. I flew to Santorini, took a 2.5 hour afternoon ferry to Naxos, then flew back to Athens. On Santorini, I stayed in Firostefani, central on the coast, more affordable, also great views, restaurants, etc. I took a day-long bus tour around the island, including winery stop, guided Akrotiri tour, lunch stop at a beach, and time in the quaint small town Megalochora, then Oia for sunset, all for €50 at the time. Loved Naxos!