Delete podcasts you've already listened to
This one's going to be kind of hard to swallow, but it's time to delete Serial off your phone. At this point, you should just assume all parties involved are guilty. Podcasts run around 25 MBs each, so free up some space by going to Settings > Podcasts > Delete Played Episodes.
Only keep HDR photos
HDR (high dynamic range) combines the best version of three different exposures into one photo, and if you have “Keep Normal Photo” (aka your inferior original) selected, you’re essentially doubling up. Don't keep the originals. Go to Settings > Photos & Camera > switch off Keep Normal Photos.
Clear your browser cache
If you use Safari a lot, it’s likely storing a ton of unnecessary web history and data to keep you browser ever-so-slightly faster. Clear that mess in Settings > Safari > Clear History and Website Data.