In many regions, the short gardening season is half over.  A lot of early crops have already completed their cycle.  You can make use of the empty space to increase your harvest with succession planting of a variety of crops.

First, you must ensure that your choices will have enough time to mature.  Most greens, radishes, bush beans, peas, green onions, and some herbs can typically be planted mid-summer and harvested in the fall in regions where the first frost is in September/October.  There are additional candidates in regions where frost is normally in November.   The key is to check the seed packet for the combined number of days to germination and maturity.  Count the number of days from the planting day to the first anticipated frost date for your region to be sure there will be enough time to grow your chosen crop.  

Another consideration is that in the spring, seeds and plants charge forward rapidly due to warm soil and progressively longer days.   Growth of summer plantings will start out quickly but will become slower with the onset of cooler temperatures and shorter days.  Allow several extra days for maturity when calculating allowances for the remaining growing season.  Be prepared to protect tender crops should an unexpected early frost threaten immature plants.

For best results, do not plant successive crops from the same family in the same place in the next planting.  In other words, try to avoid following root crops with root crops or leaf crops with leaf crops.  If unavoidable, then replenish the soil with a good fertilizer and/or compost and be ready to enjoy more fresh produce from your garden well into the fall season.