Hello,
The weather will probably be good. Since you'll be in Greece at the end of March, beginning of April... similarly to the spring temperatures that Athens sees in March, April brings even warmer temperatures as the year goes on. The average temperature this month is 15°C (59°C), while the average low is 11°C (52°F), and the average high increases to 19°C (66°F). The beginning of April starts off with mostly sunny days, with a small chance of rain, and an average daily temperature of 13°C (55°F).
What do you mean by saying a "quick flight"? For a day trip? Most of the Greek Islands are so beautiful that you'd like to spend at least 2-3 days on each of them... However, if you just want to get a glimpse you might like going to Paros. It is small and beautiful, it belongs to the Cyclades. Santorini would be nice but 1 day is not enough in my opinion.
You could also go to Aegina, Angistri, Poros, Hydra or even Spetses that are close enough to Athens. Or even Evia.