Using role-plays or dialogues you can improve your speaking ability while focusing on specific topics, grammar and vocabulary. There are numerous dialogues on this site that you can use with friends, or in class. Here are some:
Planning a Party - Future with 'will' and 'going to'
Giving Directions - Asking for and giving directions, use of the imperative
Cooking - Daily routines and hobbies
City and the Country - Comparative form
Dental Check-up - Vocabulary and phrases related to going to the dentist
For a complete list of dialogues, use this dialogues page which lists each dialogue with the purpose and language to be reviewed.