Dictionaries can be used with exercises to help understand and access a text or simply as activities in themselves.
Select some words from the text being studied that you know are new to the students. Split the students into groups and divide the words between the groups. Students find the meanings of their words, write definitions and examples and then explain them to their classsmates.
With advanced students, you can split the class into groups and give each group a page number in the dictionary. They then choose words on that page and make sentences using the words. This can be extended by students placing a blank where the word is. They then give the sentences with blanks to classmates who try to guess the word (they can be given the page number or the missing words to help them). If you choose this second option, do some examples yourself first which the class try to solve.
Students need to understand common abbreviations used in a dictionary, eg adj, vb. You can write these up and students try to guess the meanings or you can provide the meanings and students match the abbreviations with the meanings. You can follow this up with exercises such as "what is the adjective associated with season?" - students use the dictionaries to find the word seasonal.
Again with advanced students, you can split the class into groups. Give groups unusual words or words they don't know and play call my bluff. In their groups, they write three definitions for each word, only one of which is correct. They then read out the definitions to the other students who try to guess which one is correct. This also encourages students to paraphrase the correct definition. If they just copy it from the dictionary it will stand out as being the correct one. This activity could take the form of a game with points being awarded for correct guesses.