explore rival territory?

Well no spy units for the time being but you can explore their territory if you sign a non-aggression pact with them.  It's certainly not perfect but it does the job since those pacts have a duration.

no spy units, use the spell 'reveal' which reveals tiles wherever you like.

there is a summoned unit called a spy

There is also the Reveal spell, but at 10 mana for a 25-tile reveal, that gets pretty expensive...