April is not a great time for hiking and water sports unless you are in the southernmost part of Europe - Greece/Italy
A few suggestions for what you like to do:
1. Paris - great food, awesome art, great sights and really great nightlife.
2. Amsterdam - same as #1 at a smaller scale. really great nightlife, safe and welcoming.
3. Have you considered a side trip to Israel? April is nice, visit the dead Sea, great beaches, awesome food and great nightlife especially in Tel Aviv. It is a short flight from Turkey
4. Barcelona - also great food and nightlife. great beaches and small towns around BCN but the water may be cold in April
5. Switzerland - Zurich is a great getaway for nature trails, hiking and travel in the mountains - a little cold and may be snowing in some places.
6. Vienna, Salzburg - good is good, sightseeing and great nature around Salzburg; again, a little cold in April