Learn Russian: Давайте организуем пикник, это будет весело!

When making a 'Let's...' suggestion, the verb form depends on the aspect. For perfective verbs (focusing on a completed result, like organizing this specific picnic), we use Давай(те) plus the 'we' form of the future tense. If we were using an imperfective verb to suggest an ongoing action, we would use the infinitive instead.

Is организовать perfective or imperfective? How does it form the future tense here?

Организовать is a 'bi-aspectual' verb, meaning it can act as both imperfective and perfective depending on the context. Here, it functions as a perfective verb because you are proposing to complete a specific, single action. Because it acts as a perfective verb, its present-tense conjugation (мы организуем) automatically forms the simple future tense.

Why doesn't the word пикник change its ending here?

It is actually in the accusative case because it is the direct object of the verb. However, as a quick refresher, for masculine inanimate nouns like пикник, the accusative form is exactly identical to the nominative dictionary form. If you were organizing a meeting (встреча, a feminine noun), it would change to the accusative встречу.

Why do we use the adverb весело instead of an adjective like весёлый?

In Russian, when you describe a general situation, environment, or state using the pronoun это (it/this), you use the adverbial form ending in -о. Therefore, это будет весело is the grammatically correct and most standard way to say 'it will be fun'.