中译英：我们一到大理，就迫不及待地去看大理古城

once we arrived in 大理，we could not wait to visit the ancient city.

仅供参考，not sure whether it is correct

We went to have a look at the old Dali City as soon as we arrived at Dali.
We could wait to see Dali Gucheng as soon as we arrived at Dali.

用as soon as 句型或者用at the moment或用on +doing表示 一··就
We went to visit ancient cite of dali as soon as we got there.