Learn Spanish (Latin America): El encuentro es mañana en el centro.

Normally, we use estar to talk about where something is located. However, there is a special rule for events (like a meeting, a party, or a concert). When you are stating where or when an event takes place, you must use the verb ser. Since el encuentro is an event, we use es.

Since this lesson is about quedar en (to agree on), how could I rewrite this to say "We agreed to meet tomorrow downtown"?

You could say Quedamos en encontrarnos mañana en el centro. Here, the periphrasis quedar en is followed by the infinitive encontrarnos (to meet each other) to show exactly what action was agreed upon.

Could I use la reunión or la cita instead of el encuentro?

Yes, but the nuance changes slightly. El encuentro is a general word for a gathering, get-together, or encounter. La reunión usually implies a more formal or professional meeting (like at work), while la cita is a scheduled appointment (like with a doctor) or a romantic date.

Why is "downtown" translated as en el centro?