Spielzeug ist hingegen sinnvoll
Duolingo states that this equals "However, toys are useful." If we're dealing with plural toys, shouldn't the correct usage be "sind", not "ist."?
"Spielzeug" is a collective noun. The plural form "Spielzeuge" does exist, but it's fairly rare. http://www.canoo.net/services/OnlineGrammar/Wort/Nomen/Numerus/Singular.html#Anchor-Sammelbezeichnungen-11481