Learn Swedish: Den nya floristen rekommenderar en vacker bukett. - The new florist recommends a beautiful bouquet.

You would typically say Vad rekommenderar floristen för bukett? In this structure, the phrase vad ... för (what kind of) splits around the verb (rekommenderar) and the subject (floristen). You can also keep it together at the beginning of the sentence: Vad för bukett rekommenderar floristen?

Could I just use vilken instead of vad ... för to ask about the bouquet?

You can, but it changes the meaning slightly! Vad rekommenderar floristen för bukett? asks "What kind of bouquet..." (for example, are we talking about roses, a large one, or a colorful one?). If you ask Vilken bukett rekommenderar floristen?, you are asking "Which bouquet..." meaning you want them to point out a specific bouquet from a limited selection.

Why do we say den nya floristen with an -a on ny, but en vacker bukett without an -a on vacker?

This is a great refresher on how Swedish adjectives change based on whether the noun is definite or indefinite. Den nya floristen (the new florist) is definite, which requires the -a ending on the adjective alongside the double definite structure (the freestanding article den + the noun suffix -en). En vacker bukett (a beautiful bouquet) is indefinite singular, so the adjective stays in its base form.