I visited Europa Park this easter holiday and the queues were very short to none existent. But I guess the queues are picking up the further into the season it gets, so can't really give any advise on that.
I do have a couple of tips though :--D
1) Silver Star has a very short front line queue, PICK IT! The attraction is rubbish in any other seat (no air time) but awesome in the front (you reach the top before the trims hit)!
2) Do not miss out on "Pirates in Batavia" which is quite hard to find. A massive indoor ride with great amount of detail. The best one of it's kind I've ever been too!
3) Eurosat is awesome! Simple as that
4) The sandwich stall near Atlantica Super Splash serves some really nice food! The rest of the food we ate at the park was average, but this was great :--D
5) Rust is a beautiful little town to walk through and has many really great restaurants, it's great just to chill out in the town after the park has closed
6) You can buy your park tickets at most of the hotels in Rust, meaning you don't have to stand in line at the main gates. Our hotel also offered free parking outside the park!
7) Enjoy your stay, it's an awesome park cool)