If i am not considering the present economic situation in the KSA, i would say retail is a good idea.
1. You can either sell products for a commission or brand an existing range of product, which are abundantly available and create an identity, this can be done in most of the technology products, or fashion related products.
2. There is another business model i have been thinking of a way to launch, Shipping fulfillment business just like Fetchr, concentrated in one cities primarily, probably Jeddah or riyadh and grow, it has good demand in the middle east right now, and in the future of course.
3. A used products marketplace, probably one like dubizzle or olx, i know there are much, but still people are using expatriates.com classifieds mostly. If properly marketed and designed, it has potential.
4. The best of them all, An online phone repair business, i know the present situation in saudi arabia is not good for mobile phone industry, and this kind of get the phone - service - get paid - give it back to the customer as a cash on delivery or cash prepaid will have good potential, if properly marketed. I did have a website structure for this, unfortunately, i did not want to break the law by employing a non-saudi guy as technician.
Of course, the Saudi Economy is struggling now, investing heavy by expatriates is not a good idea as wide Saudization is going on.