Games that aren’t open world but aren’t exactly linear either. E.g Mass Effect. You have the main missions that are done in a linear order. But there are hubs such as the Citadel, Omega, Tuchanka where you can buy things, talk to people, have a drink and collect assignments to be done at your leisure. The Normandy (another hub) has the Galaxy map where you can pick whatever planet you want to go to. The planet is a linear mission that cannot be returned to, but again there is no order to do them in and they are optional.

Another example being the Tomb Raider reboot trilogy (especially the last two). There are many hub areas that have challenges to complete or loot to collect but the game is linear for the most part. But in these hub areas there’s plenty of side content to do e.g the Baba Yaga DLC in Rise of the Tomb Raider or various optional tombs to explore.