Word Hippo is a cute thesaurus/dictionary/word rhyming for younger learners. It asks the question “What’s another word for” at the top of the screen and children can type their sought-after word into the search space. The search at the top of the screen changes based on the tab. When on the Antonyms tab, it will ask “What is an antonym for ___?”
This site could also be useful with older students working on rhyming and poetry in Language Arts class.