Cognates give the biggest return on mental investment for language learning. This is an extensive list of 475 adjectives. Using this list will dramatically increase your students' vocabulary with very little effort - your students will be amazed at how many Spanish words they can link to the English equivalent.
The list includes level one type cognates such as adorable and responsable. The list also includes higher level vocabulary words, such as ambiguo / ambigua (ambiguous) and caprichoso / caprichosa (capricious).
Customize the list to suit your needs - use as many or as few as you wish. You might also want to have a grade-boosting spelling test where the students have to spell the word in Spanish. Just select some words that have similar (or even identical) spellings to make sure that the students know what you hope they do. (My students really appreciate a 25-point quiz to strengthen grades.)
The most important aspect is that you show your students that English and Spanish have more in common than they realize. These cognates will reinforce the idea that learning Spanish is completely do-able.