With any of the side mission achievements, make sure to check the tabs on the map screen. Press RB and go to Side Stories and check the map for any characters you may have missed.
In order to go to other previously explored islands, simply go to the map screen again and press Y to choose which one you want to go to.
Easily the best way to get memory tokens is to go fish with Big, preferably on the last island to get A TON of coins to trade in. You can also utilize fishing again for maxing your stats as well. Hope this helps!