Seaweed Necklace from the Maraqua plot. You can still get to the maraqua plot from the TCG pages.
You have the fruits, grass skirts and drinks from the old Mystery Island gadgadsbogen puzzle:
http://www.neopets.com/island/gadgadpuz ... type=party
A Ski Lodge stamp to represent the murder mystery where TNT all killed each other in the terror mountain ski lodge. There were also photos and books given out as prizes but you'd be lucky to find any of them now, if they still exist.
Ice totems and Wooden totems from the opening of terror mounting and mystery island.
You'll have to figure out your own bizarre items to go with the Mango Pango Mystery Island sacrifice plot. I don't even think most of these guys even work for neopets anymore.
http://www.neopets.com/island/sacrificers.phtml
The Mystery Island Volcano/Fire Faerie plot:
http://www.neopets.com/island/volcano_solution.phtml
Juidah's cooking pot is a R97 piece of furniture, in addition to the items you had to have to put in them.
The origina Meridell PLot:
http://www.neopets.com/medieval/plot.phtml
The second Meridell Plot:
http://www.neopets.com/medieval/plot_bfm.phtml
There's all sorts of items realting to Kayla and the other main plot characters. Just search neoitems.
Here is the Neopetsv2 plot:
http://www.neopets.com/space/neopetv2.phtml
Old Coltzan's Crown Hidden Desert Plot:
http://www.neopets.com/desert/crown_solution.phtml
HatIC plot:
http://www.neopets.com/winter/plot_hic.phtml
Another good place to look for old plot items is in the Neopedia, which has a few articles that expound on characters from plots.