(this can be completely fictional, although If you would like to use real memories, feel free too)

We have one chance to relive our most cherished memories. Due to the nature required for time travel, things will not be exactly as you remember them. Your experience could be rather distorted, twisted and strange. To initiate the time travel, picture your most treasured moment in your mind. Then you should be taken straight there. Once you've done this, you must explain what it is you'd hoped you would have been taken back too, and how it has changed.

Changes in your memory can be explained by events that have occured since that particular time in your life and anyone participating can attempt to guess/explain the reason for those changes to the original memory.