Hi, SallyCyclist,
Welcome to the forum. The only city of any size between Leon and Santiago is Astorga, which is too early in your journey, I assume. It's a very nice place, though, with some interesting things to visit (Gaudi palace, Roman ruins, plaza mayor, etc). Molinaseca and Villafranca are also both places with a nice feel and would have a bit more to do than O Cebreiro. (Villafranca is closest to your halfway point, right before the ascent to O Cebreiro and it is a very nice town).
O Cebreiro is pretty much the midpoint, and you're right that its location at the top of the climb would make for a nice resting point. It is a small, prettified, touristy destination. If the weather is nice, the views are lovely. The church is also an emblematic camino destination. IMO, there is no less to do there than there is in any of the other nearby places that would make sense, like Triacastela. I suppose you could continue on to Samos with its monastery. I am not a fan of the interior of the monastery or its paintings, but the service at night was intimate and atmospheric.
Buen camino, Laurie