Tips by Darren Rowse, summarized with permission.
Good photographs explore ideas, feelings, experiences, etc. on a deeper level. A good story made of photos will contain "plot shots" that explore themes and ideas!
A few ideas for themes to try:
Visual Themes
Perhaps colors or shapes come up again and again - especially fun to photograph while on vacation! An album filled with pictures that contain something orange with a blue ocean background (for example) is a lot more interesting to look at - it contains the element of "look for the orange".
Stylistic Themes
Repeat photography techniques and styles: there are different techniques in photography that, repeated, might be especially interesting to look at! Remember: repetition breeds familiarity and therefore comfort for a viewer.
Locational Themes
When on vacation, it can be a lot of fun to pick something cool to photograph repeatedly. For example, when Darren was on a trip, he decided to photograph markets in every city and town he visited - looking at the pictures later, it was fun to compare the similarities and differences! Other ideas might include local signage, park benches, nature, etc.