I grew up here as a kid and the best things to do is go to Parkway Plaza Mall. It is suburbia at its best. There are a few movie theaters. They have been trying to make the downtown area more exciting. Stay away from certain ghetto areas. Rancho San Diego is the better part of the town. I wouldn't suggest getting a hotel in El Cajon unless you have family in this particular city.
El Cajon doesn't have much going on, but I did go here to hike the El Cajon Mountain trail, and if you like hiking, you will love this spot. It's not a huge mountain, but the constant, steep up and down will have you feeling exhausted in the end. The view of the valley below is breath taking, and makes driving out to this place worth while.
El Cajon has a bad rep, but there are some jewels in the form of hole-in-the-wall restaurants along El Cajon Blvd.
Usually hot and nothing super "exciting" to check out. There is a casino, a mall, movie theater and some shopping options.