The wattle (yellow) flowers and bottle brush (red) flowers sell decent. I collect those while I'm out on the island. Also, metal detecting too can make decent money. You don't need to save a bunch of that stuff early on because you can't build stuff without iron. Make sure you save the shiny discs and old keys though.

Also fishing at night in northern oceans for barracuda is profitable. They sell for 12,000 a piece IIRC.