Well I personally love doing speeches and this type of things, so here are my tips:
First thing is speech techniques. Rhetorical questions, powerpoint presentations, use of moving around the room, hand gestures, verbal metaphors... It depends on how well you can speak and how serious you want your tone to be, but you NEED good speech techniques to get band 5/6
Second thing is content. Start off with a general introductin. Put something structural in there (like a verbal metaphor) that is repeated to link the whole speech.
Thirdly What you have to do is say "what the tempest suggests about imaginative journeys"... Talk about HOW (such as the play's structure, characterisation, dialogue, stage directions, imapct from shakespeare's context etc...) it tells us what you think it says (go for the journey is how art and learning lead to social regeneration, and how the island is a metaphorical place to express this). This should take approximately the first one-third to a half of your speech. Make sure you keep linking your speech somehow, and keep in mind speech techniques.
Now your related text. From the stimulus booklet, pick something that connects with what you said the tempest says imaginary journeys are (Road not taken is perfectly okay if you've already done work/want it). Then say HOW (think back to all those notes your teacher gave you on context, techniques, etc...) it shows us this message. The point out places where this text and the tempest compare/contrast
Now your other related text. Once again choose something with a similiar message about journeys (Can really be anything...if you are lazy check the BOS resources section and they've done plenty of work for you on supp texts), say HOW it achieves this, then connect this idea slightly to the stimulus book related text, and mainly to the tempest.
Then conclude, remembering your original structural item you've got linking your speech, and end by saying something wise not trailing off.
NOW, that is all help for the speech. Judging by your post your also a bit confused about the HOW the imaginative journey in tempest is expressed. A good place to start is just doing a google search for academic essays on the tempest, and reading the ones on BOS resources, or asking a good classmate/teacher. There are a whole lot of things that take us on an imaginative journey, and just start off by saying the island is an imaginary realm itself. From there the stage techniques like flying harpies (really spectacular in shakespeare's context) to the drunken ramblings of stephano and trinculo are all HOW shakespeare shows his ideas on IJs.