keys: very handy for when you need to unlock the door to your apartment. If you add car keys you can also drive somewhere which is very helpful. I use mine pretty much every day
cellphone: it’s like a multitool with a camera, flashlight and a phone. If it’s a smartphone you can all use it to access internet. Saved my life many times when I was bored and had to look at some videos online or check the news
wallet: great for carrying cash, ID and credit cards. It’s good to have your ID with you when people ask for your name but you live in a foreign country where no one speaks Polish. You can use cash or credit cards to buy other stuff you need like a bottle opener