Try rhymezone.com. It’s absolutely essential if you want to write rhyming poetry with any hope of avoiding the obvious. Be careful, though — desperate use of rhymezone may cause groans.
Also, thesaurus.com offers a “word of the day” and fun facts, like the medical word for sunburn, “erythema solare,” and word games employing techniques magicians call “legerdemain.”
Find a more serious approach at Merriam-Webster’s thesaurus site, merriam-webster.com/dictionary/thesaurus.