Probably the worst choice is to wait until you get back in town to write the review and not tell the editor that is what you are planning to do. One way or another, you need to communicate with the editor prior to the target date for the review. Among the options are:

• Asking for an extension
  
• Writing a quick review before you leave town and clearly identifying it as a cursory review
  
• Contacting the editor to indicate that you will not be able to review the paper in the required time and suggesting someone else take your place.

Given the description of this case, the third option is probably the least desirable. Unless the extension request is unreasonably long, it is likely that the first option will be viable. Alternatively, the cursory review will at least bring to the attention of the editor some key issues you think the submission raises.