The hike up Santa Ana is really worth your time, and probably our favorite thing we did in our 4 days in El Salvador. It's a good hike that take s a bit of stamina, but nothing outrageous. Your strenuous hike is rewarded with a spectacular view of the green caldera lake, complete with sound effects of steam. hissing from vents around it.
There are options in between taking the very cheap, but very slow and early local bus and paying $80 pp for a standard group tour. Uber was available from Santa Ana, and it showed about $20 one-way. We opted for a driver arranged through our hostel, which was $20 pp. He got us there in less than half the time of the bus, and then stopped at a spot on Lake Coatepeque for lunch. Having a driver was nice in that we didn't have to worry so much about getting back for the 1:00 bus. It isn't a big deal, really, but we were a little more comfortable lingering at the summit longer since we didn't have to worry about the deadline.