Solo, Meteora Day-Trip works.
The monasteries floating on their rock towers are one of the most jaw-dropping sights in Europe, and many do it by scenic train to Kalambaka. The catch: it's genuinely far — 4+ hours each way, realistically an overnight rather than a sane day-trip — and rushing it as a 12–14 hour same-day loop from Athens means hours on a coach for a fraction of the time on the rocks.