Learn German: Die Überraschung ist uns gut gelungen. - The surprise worked out well for us.

The verb gelingen (to succeed / to work out) works differently in German than in English. The thing that succeeds is the subject (Die Überraschung), and the person it succeeds for takes the dative case (uns). You literally say that the surprise succeeded to us.

Why does the past tense use ist instead of hat?

The verb gelingen describes an event occurring or a change of state, and it does not take an accusative object. Because of this, it builds its Perfekt (past) tense with the helping verb sein rather than haben.

Could I use schaffen to say we managed or pulled off the surprise?

Yes! If you want to make 'we' the subject, you can say Wir haben die Überraschung gut geschafft. While schaffen focuses more on your active effort in pulling it off, gelingen sounds a bit more elegant and focuses purely on the successful outcome.

How would I add a nur dass exception clause to this sentence?

You can attach it at the end with a comma, treating nur dass as a subordinating conjunction that kicks its conjugated verb to the very end. For example: Die Überraschung ist uns gut gelungen, nur dass sie etwas teuer war (The surprise worked out well for us, except that it was a bit expensive).