If you want to travel to Europe, you must be able to prove your identity in EU or non-EU countries. Documents officially recognized by all European countries are the national identity card (except in Kosovo where the passport is obligatory) and the valid passport (at least 3 months before expiry). Here are all countries where you can travel with an outdated but theoretically find yours valid identity card. Think of your driving license in case you have to drive even if it is not planned.
Make sure you have taken all your papers before you leave