Hello,
My suggestion is make Islands that branch off you mane island, this way you could add boats and all types of things etc... you could make you explorer find them to. They could have lots of recources but mybe make them already have them and when you deplete all the reocurces the island disaperes and you have to find anther one. Or make them permanent just like a smaller version of your current island, and make the explorer take about 72 hours to find them, and you could have more depending on your level etc...
This is my suggestion, i hope you like it
Kerrod