I spent a total of six months in El Salvador over a period of three years in the early nineties. Like you, I was working (in San Salvador) so didn't get too much opportunity to 'tour'. Outside the city I saw mainly western ES and the highlight was the Volcan Izalco (Cerro Verde). Other well known scenic areas are the Montecristo Cloud Forest on the northern border and Lago de Ilpango east of San Salvador. I also enjoyed a trip up the volcano San Salvador (Boqueron) where this photo
was taken. The next one (My Laundry) just proves that it doesn't really matter where you are. OMG I just realized that I still have and wear some of those clothes
BTW, I would advise giving consideration to what equipment you take to ES, particularly if you travel alone or in rural areas. On my first trip right after the war I left my Leica behind (of course) and took a Canon AE-1 and consumer zoom. I walked all over town hours on end and never had a problem. But over the next several years things deteriorated to where I just took a Stylus. Again I never had a problem but a friend's wife was robbed at gunpoint in midday on a main thorofair while standing in a crowd of 20 or 30 people at a bus stop. Carjackings became common even in the 'good' parts of town. One of my co-workers was robbed at gunpoint while going up the same volcano I had previously visited. And on my second visit to the volcano I was within 100 feet of a gunpoint robbery attempt of a traveling Swiss student who had gone around the world for a year with no incident. The student had a backpack with Nikon gear inside but the robber demanded his boots at gunpoint. The student stomped the robber's bare feet with his heavy boots and ran - into us. Fortunately the robber didn't pursue and we gave the student a ride to the hostel. He was lucky to be alive.
Don't get me wrong. I absolutely love El Salvador and the Salvadoran people. It's a beautiful country and a beautiful people, but the war left a generation raised as warriors with no job skills, and no jobs even if they had skills. Enjoy your trip - I envy you and hope to return myself some time, in particular to visit the Monte Cristo Cloud Forest which I understand is spectacular. Don't travel alone, don't travel at night, keep an eye out, and you'll be fine. And as Sergio said take the opportunity to photograph people. They usually love it and are very photogenic.