If you find yourself 1. in San Diego (hard pre-req guys, sorry), and 2. searching for a just-challenging-enough hike, look no further than Cowles Mountain.
Located in the San Carlos neighborhood in the city of San Diego, Cowles Mountain houses the highest point in San Diego county, which on a clear day gives hikers views all the way to Mexico.
The arid climate and dry vegetation usually make for medium to high temperatures, but the hike is short enough that you could do it during peak sun hours (provided you are wearing sunscreen!) and feel totally fine.
Obligatory top-of-the-mountain selfie.
Sarah and I hiked Cowles when she was in town for Memorial Day, and as you can see from her smiling face (and her perfect "tree pose" in the first photo), she loved it. Another group that loves this hike? Dogs! Not only is Cowles dog-friendly, but the distance and conditions are feasible for most dogs (at least from what I've observed from many of the hikes I've done here).
Overall, I'd give Cowles a difficulty level of 5, which is just perfect for a hike that you could decide to do that morning, and the reason why I consider it to be the most reliable hike in San Diego.
What are some of your favorite hikes (either in or outside of San Diego)?