Making language relevant and not just rote drilling of rules is a fundamental way to liven up an adult class. Try an everyday activity like going to a movie or ordering food in English. You can create a script for lower-level classes to practice verbatim in groups, or introduce a variety of phrases and allow higher-level students to improvise. Depending on your class, it's also a great way to promote free conversation -- you can discuss topics such as cultural differences and traveling to English-speaking countries.
Adult ESL students can be shy about their ability level, even with one another. Help your students get to know you and their classmates and practice English at the same time through ice breaker-style activities. Try having students ask each other about pop culture favorites and see which classmates they are similar to. You could also try a game where you gather a pool of students' facts about themselves and the class tries to guess who each fact describes.
A variation on scripted dialogue that is well-suited to adult ESL students is role playing exercises. These are particularly valuable for business-oriented classes or students, where you could role-play many important aspects of business from the job search to visiting a foreign office to international conference calls. A practice job interview is a great way to introduce role playing activities. The interviewee can draw on his own experience, or make up a character with different qualifications, and the interviewer will learn common phrases and terms in business English.
Beyond role-playing activities, business situations present other real-world opportunities for adult students to practice. Even if your adult class is not business-specific, you can get your students to use practical phrases through an activity where they describe their job or company to you. Introduce the activity by demonstrating phrases like "(my company) employs 100 people" or "is in the manufacturing field." This is a flexible activity for any skill level, and it can also serve as an ice breaker for free conversation about work in the classroom.