Almost finished at the end of day 3. What now?

Even if that happens you cn burn the days on a Inn no problem , but yeah soem new quests opens up in all the regions. About preparing well .... on the extra days new Schematas are for sale and also do exintct the mobs as the best accessories and weapons are drops from the last ones.

@grantheaslip: All of that done before the end of day three? And on your first playthrough? Wow.

I guess my advice is clean up on all of the side quests that you can, then spend time exterminating as many monsters as you like. You're probably not going to be tough enough to take on Aeronite (the game's optional super-boss), but with that kind of progress, you are going to unlock the game's ultimate dungeon. Exterminating certain monsters like the Reaver is especially beneficial because of the item drops the Last Ones give (Reaver's being a powerful weapon). Exterminating monsters also makes the going through the bonus dungeon an easier affair and will also help during an optional (but highly recommended) objective in the endgame dungeon.

If you are absolutely left wanting for things to do and feel your equipment and items are sufficient, using inns to push time ahead is perfectly valid. But my first recommendation is to use as much of the time you have left to benefit yourself in whatever way you can before you start burning down the clock.

If you're going for platinum. Don't forget to get all of the seeds. it's easiest just to walk back and forth along the highways to pick them up. But if you forget and wait too long you wont have enough time to pick them up. You need 100 total and you can only get like 15 in a half day.

also, yeah kill all of every monster (except that metal bug thing, it takes like 20000 kills). that'll make the extra dungeon easier.

it's also possible to kill Aeronite but you'll need pretty specific strats to pull it off. I recommend googleing/youtubeing it.

I platinumed on one playthrough plus just a bit of cleanup. sounds like you're off to a good start.

Aside from the final boss and the bonus boss, the mandatory encounters you have left are relatively trivial. You shouldn't take the Last Ones lightly, as a number of them can pack a surprising punch if you're not prepared, and they may require altered tactics.

The final boss is a multi-stage battle that serves as part endurance test and test of your staggering ability. There is no one strategy that will win out against it, and it is possible (but very, very difficult) to get through the final form without performing a successful stagger. I ended up winning by constantly using the Spira's Summoner garb's Elementa ability while keeping several debuffs active as I could, switching to other garbs to defend, and then during the final form, overclocking and using Elementa like crazy to both stagger and finish the boss off. It helped that I had a couple of Ethers on hand in my inventory, but again, the way I won is by no means the only way to do it.

Also, seriously, do the final optional quest in the last dungeon. It grants you the most powerful weapon/shield combo you can possibly get on the first playthrough and will help immensely in the final battle.