When you are planning speaking activities, you need to consider a number of stages, these are 1) a warm-up activity, 2) review of the language needed, 3) whole group practice of the activity, 4) speaking practice and finally 5) the feedback stage.
Stages 2 and 3 can be incorporated. I've given a brief outline below to help you see how to structure a lesson.
You could use pictures to practise structures of inviting someone out, accepting and declining. For this you will need pictures of different social activities eg cinema, ice skating, restaurant, bar etc.
Make sure the structures of inviting, accepting and refusing have been presented and practised in a previous lesson. In the lesson itself you will need to revise and activate the structures.
To warm the students up to the topic, you can use a structure such as "fun activities to do at the weekend" to play hangman. When they have guessed the structure ask them for examples of such activities.
Then invite one of the students out with you, showing the corresponding picture. For example, "would you like to go to the cinema with me?" and encourage the student to answer. At this point, you should indicate that the reply is correct or give the correct version yourself and have the student repeat it.
Continue inviting other students showing other pictures. The focus here should remain on accuracy so that the students know how to use the phrase before they work alone. You should then write up some of the most common phrases for accepting and refusing invitations. Plan beforehand how you will structure the whiteboard or paperboard to enable you to leave the language on view for the duration of the class.
When you think the students have got the hang of the activity and are producing the replies correctly (or more or less), indicate to one of the students that you want him / her to invite you out. Focus on the correct usage of the invitation. Again, note on the board the most common structures for inviting someone out. Indicate to other students that you want them to invite you or classmates out. Throughout these presentation activities, you should focus on the correct pronunciation, marking stress and intonation patterns on the board.
Depending on the level of your students you can decide whether to incorporate any of the following: a review of prepostions to use with days and times. Phrases for making alternative suggestions. You could also focus on levels of formality. For the following speaking activity, you can determine the level of formality, if you feel it is appropriate.
You can then put students into groups, tell them they will be spending some time together over the weekend. They decide what activities they will do together. You can give them diaries with some time spaces already filled in to make it necessary to practise refusing. If you leave it open, they might be tempted to accept everything.
While students are speaking, you can go around noting good points and errors.
At the end of the activity, students can present to the class what they have decided to do and when.
You can then feedback on elements that were particularly good and indicate errors that you noticed and invite students to correct them.