Finished the main story but I don't wanna be done with the game yet.

Anything of interest I should be doing? I hear things about an awesome optional dungeon. Any other good quests to do though? How do I beat the giant turtle??

Any suggestions would be helpful

Go talk to Ezma at the Hunter HQ to unlock the Menace Dungeons, get the legendary weapons from Randolph and get all the Royal Arms if you haven't already.

I would save killing the turtle for last honestly, it takes a long ass time and doesn't have a ton of pay off. If you want to take a break from combat you could hit up the Pitioss Ruins first, keep in mind it will probably take you about 5 hours to complete it on average. There is zero combat in there, it is all platforming and puzzle solving. Other than that there are the hunts and the secret dungeons. To get access to the secret dungeons you need to have completed all the other dungeons, namely Costlemark. Best thing to do would be to look up a guide to the post game just to know where to go.

Go near the first tomb north of hammerhead and follow the base invasion side quest that triggers up there. Two of the best/weirdest things in this game follow from that.

yeah for sure do the fallmouth garrison first priority. if you need to level up just chain together as many hunts as you can and then rest in galldin quay for the double xp.

If you are going to fight the Adamantoise eventually, you should do the weapons quests first. Getting the Zwill blade will make the fight a lot faster, probably reduce the time from 1-2 hours down to 20-30 minutes. There are people who did it in less than 8 minutes.

Also if you are going to 100% the game it is highly recommended that you start learning playing the game without relying on healing items. SPOILER ALERT There is a late game dungeon that disables item usage. Also Pitioss Ruins is a super well designed dungeon.

Pitioss Ruins. You get a really good item that can be beneficial for the endgame dungeons and the missions for those legendary weapons.