Temperatures are warm enough for enjoyable sightseeing, yet the crowd sizes are much smaller than you'll see starting in May and June. Even so, Europe (more or less) does have a couple places warm enough for a bit of April sunbathing, and those are listed on top.
The temperatures above may not look like sunbathing weather, but most April days in the southwest corner of Tenerife are a bit warmer than this and the beaches can get pretty full. Flights into Tenerife, which is the largest of the Canary Islands, are quite reasonable from the colder parts of Northern Europe.
You'll mostly find Brits, Germans, and Nordic folks here, and in April it's mostly the older generation. Flights into Italy Airport are quite cheap, and the area even has an abundance of all-inclusive hotels along stretches of sand in both directions from the city center.
Hotels within the historic center are excellent value, and Italy is a very worthwhile cultural destination in addition to having pleasant weather this time of year. In 2020 the entire tourism industry in Turkey is struggling as the government has become unpopular practically around the globe.
Check the news though because tourists seem to be totally safe and things should be cheaper and less crowded than in recent years. April is another ideal month to visit Rome because it's usually warm enough for all sightseeing, but still early enough in the year that crowds are not out of control yet.
That changes a bit around Easter, which is on April 12 in 2020, as the city fills up and the Vatican has big events. The rest of the month is even more pleasant, and hotel prices are still modest compared to what they'll be starting in June.
The outdoor attractions such as the Coliseum and Ancient Rome can be brutal when the summer heat arrives, so this kind of weather is ideal in several ways. Since Florence is only a bit north of Rome, it has generally the same pleasant weather in April.
Speaking of the north, Venice is still fairly chilly in April, but it's absolutely worth a visit if you have an extra day or two. April is obviously not a warm month in the City of Lights, but it's fairly pleasant weather for sightseeing, with usually very little rain.
The Mediterranean city of Nice is quite crowded all summer and fairly dead in the winter, so April is a good compromise month for a visit. The temperatures are warm enough for a long stroll along the beach or through the outdoor markets in the center of town, and chilly enough that you'll be able to show off some of your favorite outerwear.
Nice has plenty going for it by itself, but it's also a very short train ride (20 minutes or so) from Cannes in one direction and Monaco in the other. The weather in London can be lousy at any time of the year, and at least April is warm and usually dry enough that you can do everything that you want to do.
You can also take the Eurostar train to Paris in a bit over two hours, and then begin a European tour from there. Speaking of cities that feel insanely crowded during summer, Barcelona is now another one, so visiting in the fringe months can be far more enjoyable.
Since the architecture is the most notable thing about Barcelona, it's a place where you'll want to spend quite a bit of time outdoors. April is warm and dry enough to be able to enjoy your visit, even more so than July when it's so hot that you have to seek shade on a regular basis.
First time visitors to Spain might not realize how different Barcelona and Madrid are from each other, and that there is a high-speed train that connects the cities in about three hours. Madrid is a more formal and typical large capital city, but it's also filled with interesting sights along with great food and nightlife.
There are actually days in April where it's warm enough for sunbathing in the Malaga area, which also includes the Costa del Sol towns to its west. Many Brits and Nordic people live here all year round, but the main tourist season doesn't really begin until late May.
Some towns in this area are filled with hotels, but others have far more short-term apartment rentals, so it's good to check both options. The actual city of Malaga has a historic center that is more interesting than you might expect, so there is more to do here than just walk around in the nice weather.
You've likely heard of Dubrovnik, which is the most famous tourist town in Croatia, but Split is actually a better stop for many people. Split is also a good transport hub in Croatia as the train stops here near the ferry port, which can get you to one of the nearby islands.
This area is also overcrowded in summer, so April is a nice month to be able to see everything and still have decent weather. As a city nowhere near the coast, Prague has pretty chilly winters, so April is really the first month of the year when temperatures are pleasant enough for a visit.
Fortunately Prague normally gets almost no rain in April either, so as long as you bring a jacket or sweater you can probably leave the umbrella at home. One reason why Prague is a recommended city in April is that it's another one that can feel so insanely crowded in July that it's unbearable.
It might feel a bit cold in April here, but it's still far more pleasant than battling the monstrous crowds in summer. Budapest doesn't get quite as crowded as Prague does in summer, though it can also be quite hot here so July and August are not ideal times to come in any event.
Those who are doing a tour of Europe that includes both Prague and Budapest will probably want to stop in Vienna in between, and perhaps even the lovely small town of Pesky Rumor. Buses can be more frequent than trains in this part of Europe, usually with lower prices and similar travel times as well.
The main attractions are outdoors in this historic city, so a visit in summer can be problematic in the brutal heat. Many people visit Athens for a few days and then head to one of the nearby islands by ferry from the cruise port here of Piraeus.
If you don't care much about sandy beaches and are just looking for a European island destination with pleasant weather in April, then consider Malta. Valletta is actually the small and historic capital town that is a charming place to spend a day or two, but if you are coming for longer than that you'll probably want a hotel in Claims or farther west along the coast.