Your router protects every device in the house, and it's probably the one you've thought about least. Work through this list once and you're done for a year.
Change the admin password
This is the password for the router's settings page, not the Wi-Fi password. Many routers still ship with a default printed on a sticker. Change it to something long and save it in your password manager.
Update the firmware and check the encryption
Look for a firmware update option in the router settings and turn on automatic updates if it exists. Then make sure the Wi-Fi uses WPA3, or WPA2 if some of your devices are older. Anything labelled WEP or "open" needs fixing today.