Croatia in Early April

Where are you flying out of - Zagreb too? If so, I might be tempted to fly immediately to Split or Dubrovnik and work your way north back to Zagreb. (Croatia Airlines flights within the country are pretty affordable.) But you really don't have much time, so going all the way down to Dubrovnik might not be the best idea. (Does 7 days mean six nights in country? I always plan my trips by nights, not days.)

Let's say you have six nights and are flying out of Zagreb. Then I might try to head down to Split, spend a few days there, rent a car and drive north, stop at towns like Trogir, Sibenik, and Zadar and stop for a night at Plitvice Lakes National Park. Drop the car in Zagreb at the end, spend the last few nights there.

Or, you could split your time between the Slovenian capital of Ljubljana (train from Zagreb) and Zagreb. Do a day trip to Plitvice Lakes from Zagreb - and skip the coast. It might be rainy in early April, so you might want to be in the cities in the rain - at least you can duck into museums until the rain stops. Pick a nice day for the day trip to Plitvice.

In April, you probably need not book much in advance as far as hotels and B&Bs ("sobe"). beyond your first night, If you have an internet device like a phone or tablet, you can book as you go along and change up your plans on the fly if you want (easier if you have a car). If the weather is good, drive to the coast; if not, stick to the cities and towns.

Don't be stressed about the time you are visiting - I think April is a great time to visit Croatia. Here are some insights into croatian weather https://mappingcroatia.com/weather-in-croatia/ . The only thing you will miss is beach time but there are so many other advantages, no crowds being No1. As far as your itinerary it really depends if you want to have a rest or see as much as you can in a limited amount of time. Luckly Croatia is small enough you can try to combine both. I suggest you focus on two or three main destination like Zagreb, Split and Dubrovnik and do the rest of sightseeing on the way.

In order to see as mush as possible and still take it slow I would do the following:

Day in Zagreb
Day trip from Zagreb to Slovenia
Zagreb to Split with stop at Plitvice Lakes
1 or 2 days in Split
Split to Dubrovnik across Mostar (Bosnia & Herzegovina)
2 days in Dubrovnik
flight back to Zagreb

If you don't want to fly back to Zagreb I suggest you skip Dubrovnik because it will just be to exhausting.

We go to Croatia every April for many years now by choice.

My 7 day itinerary would look like this:

Day 1 - Zagreb, sleep Zagreb
Day 2 - Morning, explore Zagreb, afternoon travel to Plitvice, sleep Plitvice
Day 3 - Plitvice, sleep Plitvice
Day 4 - Travel to Split; sleep Split
Day 5 - Day visit to Trogir; sleep Split
Day 6 - Explore Split; sleep Split
Day 7 - Fly Split to Zagreb and to home

The local 7-day itinerary would look like this:

Day 1 - Zagreb, a few hours to see the main square. Take a bus or rent a car and go straight to Plitvice to take fresh air. Private accommodation is very popular and cheaper on D 52.
Day 2 - Morning, explore Plitvice and maybe taking one more day.
Day 3 - Travel to Split. Not so far from D 52 on the way to Split visiting Nikola Tesla Museum.
Day 4 - Explore Split; sleep in Split until the end vacation or catching same day speed boat to the south island for one night and back to Split the next day. Croatia is all about the islands. You should visit at least one island. Hvar or Korcula.
Day 5 - Day trip to Krka, Bol, Blue cave, Omis, Makarska.
Day 6 - Day trip to
Day 7 - Fly out from Split (Split has the direct flight to bigger German airports for connection)