The most prized posessions, even among Lower Tunnel rats, are human curios. Objects that might not have any real practical use, but hold an almost mythical one to the rats. Even something as simple as car keys could be considered an artifact worthy of studying, and owning.
The rats seek more practical goods as well, and most often what all rats want is something soft, and colorful. Clothing is not elaborate in Mega Burg, but given the choice of wearing something bright yellow or brown the louder more obnoxious color would be preferred. Shiny objects, gold or silver, are also favored, to be worn as ornaments or jewelry.
Spices, especially those that greatly change the taste of day to day meals are also on high demand. A packet of spices (salt, pepper or something more oriental) could impress many guests, while also bringing much satisfaction to the owner as well. And since most spices have a very long life, and can be stored in dry packets (unless exposed to water) it is akin to silver or even gold for humans.