In addition to utilizing a postcard printing service to make summer postcards, you could also order and install a new line of taxi toppers to target the tourists visiting your city throughout the summer. These are a perfect way to reach the summer market, as people who are from out of town tend to rely on taxicabs as one of their main methods of transportation.
In cities like New York, the taxi topper tents are illuminated, allowing your ad to stand out during the day, but to shine even brighter at night when more people are out attending a Broadway show or enjoying the Brooklyn nightlife.
You may also want to consider posting custom posters near car rental agencies, since these tend to be a second mode of transportation for tourists. However you decide to target this new, temporary summer audience though, make sure you design your new marketing so that the images are sleek, summer appropriate and will give potential customers all of the information they need quickly and simply.