Excellent ideas for helping students retain vocabulary. Some are content specific (i.e., scientific words) and yes, I think adult students are past coloring :-), but there are other options in here which would work with adult students.
This is a vocabulary learning platform with phonics. It has many, many vocabulary categories to choose for lower level vocabulary (classroom instructions/ parts of the body) to more advanced vocabulary (character traits, natural disasters, solve the crime). It has matching quizzes for each category. Definitely good for lower levels 1-4.
good at home study site, especially for lower levels (even pre-1!). Has vocabulary, grammar, pronunciation, etc. with a strong focus on beginning survival vocabulary.
A very, very cool site -- suggested by Mike, thank you! -- that allows students to look up a vocabulary word and get various perspectives -- definitions, example sentences, and best of all -- pictures, even for abstract words (I used "attitude" as my test word). Great resource for higher level students.
Simple and easy technique to help students deal with the "look every unknown word up immediately in the dictionary" state of mind. Short, simple, and requires NO tech tools other than a pen or a marker!
I found this awesome vocab tool that picks out important words in an online (or typed in) text based on relevance, frequency, and subject. It creates word clouds (don't worry, I didn't know what those were either), gives definitions, parts of speech, and examples from the text. The full features are only available with a paid subscription, but there's a lot you can do with the free version, too, and you don't even need to create a username or password! This could be a very helpful website to pass on to students.