In April the European temperatures begin to climb, especially in the south.  Perfect time for a city trip. It is not too hot to explore the city and it's usually not that busy. Moreover, as it’s not the high season (unless you are traveling during the Easter holidays), prices are reasonable. Here are a few suggestions:

Cyprus

The temperature is already rising to a pleasant 24 degrees. With a little luck you can already lie by the pool. Be sure to book a hotel with a heated pool. It is also not too hot to soak up some culture. Cyprus has many well-preserved ruins that are worth a visit. In the Troödos mountains you can take beautiful walks. All plants are now in bloom, so it's the best time of the year.

Cyprus 2003360

Georgia

Temperatures can reach 20 degrees already. Near the Black Sea temperatures are highest, but they have more rain. In the east of the country temperatures are more extreme with cold winters and hot summers. Culture lovers need to visit the capital, Tbilisi. In the south of the country you will find the beautiful monastery Davit Gareja. If you want to hike in the Caucasus, you better be patient because in April it’s still quiet cold.

Tbilisi 683944

Madeira

This Portuguese island has a mild climate. In winter it rains quite a lot, but from April temperatures are pleasant, and it's still pretty quiet. Nature is beautifull and everything is in bloom.

Madeira 2009313

Spain

Temperatures rise to 25 degrees in the south of Spain. This region has a lot to offer. In the summer I find it too hot to visit cities such as Malaga and Seville. Spring is the ideal time. Less crowded and pleasant temperatures. 
The Canary Islands,Ibiza and Majorca are all perfect destinations to get that first tan.

Plaza Espana Sevilla

Where are you going in April?