Definitely, visiting Athens, Delphi, and Metéora can be an incredible journey through Greek history and culture. Here are some tips for your travel:

Best Time to Visit:
The best time to visit these places in terms of weather and crowd density is during spring (April to June) and fall (September to November) when the weather is pleasant, and there is less tourist traffic.

Route and Accessibility:
Athens is well connected by the airport, and you can easily take domestic flights or buses to Delphi and Metéora. You can also hire cars if you plan on driving around yourself.

Altitude and Acclimatization:
Athens, Delphi, and Metéora aren't located at high altitudes, hence altitude sickness isn't something to be concerned about.

Accommodation and Booking:
Accommodations range from budget hostels to luxury hotels in all three places. It's advisable to book your accommodations well in advance especially if you're traveling in the peak season.

Clothing and Essentials:
While your clothing would largely depend on the weather during your visit, it's recommended to wear comfortable shoes as these locations involve a reasonable amount of walking and exploration. Remember to carry sunscreen, a hat, and sunglasses to protect against the sun.

Health and Safety:
Ensure to have travel insurance before your trip. Keep yourself hydrated and take care of your belongings.

Local Etiquette and Respect:
Greek people are warm and friendly. When visiting religious sites, dress modestly. Try learning a few Greek words; it's appreciated by the locals.

Remember to respect the age-old monuments, maintain the cleanliness, and strictly follow the guidelines provided at all the historic and cultural sites. Enjoy your wonderful trip to Greece!