Where I work, we have basement secure bike parking, a permanent locker and free showers and towels. I reckon if you are serious about promoting cycling that's the bare minimum you should be providing. It's common for people to bring clothes in on Monday for the week and I also need somewhere for toiletries (cleansing, makeup, hair) so a permanent locker is essential. I do have a locker in my office but it's a trek to the lift and then up a few floors so not really practical. We also have a drying room, but I only went in there once. It was rammed with stuff that people had left and never returned for and it stank to high heaven. If my jacket is wet, I'll dry it on the back of my office chair I'm afraid.
I do like the idea above about having lockers with usb sockets for charging lights. We have those in our office lockers and it's really useful. It would be great to have a track pump too, although I carry tools, a pump and a spare tube with me. I'm not sure I care for these things so much though that I'd pay extra. Bike washing / servicing would be worth it though as I keep my bike stashed at a mainline station so rarely have the opportunity to clean / maintain it.
One thing I'd say is ensure the access to the bike racks is reliable. We have two lifts in our building, but one has been out of service for several months. The other has been out of order twice in recent weeks, causing chaos. I've had to carry my bike down several flights of stairs and squeeze it into a small service lift to get it back up again. A guy from another office in our building had to leave his motorbike there the other week because there was no way to get it out

.