Learn German: Ein kurzer Schauer wird das Picknick nicht zerstören. - A short shower will not destroy the picnic.

To add "unless it rains heavily" using es sei denn, you treat it as a coordinating conjunction introducing a main clause with normal word order: Ein kurzer Schauer wird das Picknick nicht zerstören, es sei denn, es regnet stark. If you use es sei denn, dass, it introduces a subordinate clause, kicking the verb to the end: ..., es sei denn, dass es stark regnet.

In English, we usually say "ruin" a picnic rather than "destroy". Is zerstören too dramatic here?

While zerstören literally means "to destroy", it is perfectly normal in German to use it for abstract events like a picnic, a mood, or a plan. Another great alternative here would be verderben (to spoil or ruin), or the common weather idiom ins Wasser fallen (to fall into the water, meaning a plan is cancelled or ruined).

Why does nicht come right before zerstören at the very end of the sentence?

In sentences with a helping verb like the future tense wird and an infinitive like zerstören, the word nicht typically goes as close to the end of the sentence as possible, right before the verb it is negating. Here, it directly negates the action of destroying.

Can Schauer mean anything other than a short rain shower?