The first caravan usually arrives with a fair amount of useless trash, bringing goods that your fort does not currently have spare of. After you specify what the next caravan should bring, you have a lot of control and potential Dwarfbucks to sink. There are a fair amount of raw materials I buy, because frankly there just isn't any other use for trade goods or 3,000+ cooked meals other than selling to the caravan, so turn the otherwise unwanted resources into Dwarfbucks into relevant materials.
I usually buy the more exotic food ingredients, as many of them are weird stuff you won't find on your embark, and the more of it you have the more likely some dwarf will find a meal s/he likes. Raw materials that are otherwise hard to acquire in large quantity, like the more expensive leathers or GCS silk thread, uncut and cut gems, books and bookmaking parts, bags of flour and dye, breeding pairs of animals (ESPECIALLY GIANTS from the elves, WTB giant grizzly military companions), and anything made of iron or steel, which can be melted back down into useful items.
Maximizing the caravan delivery is almost a topic in itself, but if you order enough gems, expensive leather, GCS silks, platinum/aluminum/steel/gold bars, etc. you can get caravans hauling a half a million dwarfbucks worth of goods. Note - except for the metal, all of those items are lightweight and high value - this lets the caravan carry more money for its weight.