I'm not sure I'd classify project euler as a C++ project. It's mostly made to enhance your mathematical notions in programming in general, and you can use whatever language you'd like to solve these problems. Though don't get me wrong, it's an amazing website and do give it a look
To answer your question I'd recommend a few things, the first one being to buy a book. They come with plenty of exercises as well as a wealth of knowledge and tips to help keep you sharp! As for a lack of imagination to make your own projects (I know that feel bro), you can go on Stackoverflow.com where a ton of users, expert and novice, post their problems for the world to solve. Kinda works like yahoo!answers (or whatever it's called) in that the best answers get voted up. I try to look at some of the problems people may be having and attempt to solve them myself. Though often unsuccessful as I am myself quite a novice when it comes to programming, it builds up your problem solving skills, something you need if you're going to stay in the field
In my opinion, nothing beats a good 'ol good. Some may disagree, it's all a matter of preference. Gave you my two cents, hope it helps! Cheers!