Since you are considering leaving your hotel and going to the airport I assume that means you are not jet lagged and are ready to drive. Thats good. We have rented from ACR and found them to be very good. They don't have an office at the airport. They park your car in the parking lot, do the paperwork on the hood of the car give you a map and a walk through on the car and away you go. I know that sounds a bit strange but this is pretty normal for local companies to operate.
Get out as early as possible. There are tours and local public buses that make the trip to Delphi daily but they stop on the way. If you drive directly to Delphi you can beat the crush of people at the site. When it becomes too crowded nip over to the museum. It is fairly small but informative. You should confirm operating hours for the site. I am not sure if April is still winter hours but if it is the site will close at 3 or 3:30 its important to know that while planning.
It will be about a 3 to 3.5 hour drive to Meteora. Plan to arrive about 45 minutes before sunset. Drive up to the monasteries and get photos from the best vantage sites. You will have the area virtually all to yourselves. Go to your hotel after sunset. Next morning get going early to beat the tour buses. Its a 5 hour drive back to Athens so you are probably going to have to leave in the early afternoon.
Here is what you will see.
Olympia Delphi Meteora https://www.flickr.com/photos/stanbr54/sets/72157645469717811/
