I know a driver that can do airport pick up and/or drop off or can be hired for an entire day. He is not a trained or licensed guide but he knows the entire country well and is very reliable. He has good cars, well maintained and he is always on time. I usually hire him for an entire day or days. He can also drive you to Guatemala if you are planning on combining the visit. His name is Francisco Rivas and his phone +503 7703 3278. He usually responds quickly via Whattsup. I have no financial interest. Just a happy customer.
Most hotels also have their own preferred drivers. I find that it is hit or miss as far as timeliness or general reliability. Enjoy El Salvador.