Hey Amanda Panda--
I think there are places to ride up near Segovia and Avila. The following two are in Avila (I've never been to either--I just found them by doing a search in Spanish):
ALMANZOR TURISMO ECUESTRE
Barajas de Gredos 5635 Navarredonda de Gredos
Tel.- 920 34 80 47 Fax.-920 34 81 14
Excursiones a caballo
GREDOS RUTAS A CABALLO
Trigueras 4
5634 Hoyos del Espino
Tel.- 92 034 90 85
Paseos y Rutas a caballo
And this one is in Segovia:
Hípica Eresma
Teléfono: 921 12 00 42 / 659 48 90 86
Localidad: Segovia
E-mail: e.camiruaga@segonet.com
Alojamiento: El Molino Viejo
Más información: Oficina de Información Turística de Segovia. Tel.: 921 46 29 06.
You can also check out a more comprehensive list (in Spanish, but you can get phone numbers and names from it!) at
http://www.todocaballos.com/countries/es/tpl/nrs/sections/rutas/ Also, check out
http://www.equiberia.com/ for some cool horse vacations (now that I have found that, I am putting it on my list of "things to do in Spain someday")!