You really can't do things in the wrong order! You can discover lots of islands and shit, but the main quest is strictly in order. There are some heart pieces on Windfall Island, if you've been there and left already just go back. Have you done Dragon Roost Island yet? ALSO, don't forget to feed the fishes around all the islands, they will mark your map. You will get sick of the fish music, though.
SPOILERS KINDA DON'T READ IF YOU DON'T WANT TO
@ Windfall Island
Play hide and seek with the Killer Bees. Look near the hippie guy, I think there's one somewhere near him. Get the pictograph, take a picture of the girl hanging out near the Killer Bees, and give it to the walking guy. There are more but I can't remember.
OH and if you find Tingle Isle there's a giant Octo somewhere near by who has a heart piece. Look for seagulls. I think the Octos are easiest to kill with the boomerang, don't know if you have that yet.