You would basically do everything minus the raids. So that's collecting Mokoko seeds (collectibles you find all over the open world), doing your dailies and weeklies (minus the raids), sailing events and do islands. Trade skills can be fun. They're pretty simple but they have a daily cap.
The islands are all very varied, and they're hit-or-miss. There are islands ranging from a Resident Evil themed island to an island with snowball fights. E.g.:
https://youtu.be/kBdWfUw8EaA?t=591
https://youtu.be/zzE695myxbo?t=233
It is really fun IMO.
BUT you will finish them at some point and you will have nothing to do because
vertical progression is gated by raids. I would only recommend players who don't want to raid to play Lost Ark if they are OK with not having the best gear and mostly doing horizontal progression. It's not a casual-friendly system. But if you don't care about your gear score that much I think casual players will have a good time. Just go in playing it for free then quit when you reach the point where you have to do raids and start grinding a shit ton (tier 3 and beyond).