Wait, what? Yep, recipes. And I don't mean just food items either. I'm talking about blueprints, schematics, and plans for technology items. They're not heavy, don't take up much room, and can be quite valuable.
Depending on the relationship between any two planets, it could be a licensing deal (ie selling permission to use for some time), or a one-time deal (permission to use forever). The latter, of course, would be much more expensive. But if the two planets don't have any kind of patent reciprocity, it's the best you can do.
Update after comments
The reason to bring the information in person (I should have expanded on this earlier) is that in many cases it will be difficult to actually implement the technology even with instructions. I'm envisioning a clan of space-gypsy general consultants who sell the information and then stick around long enough to help the customers through the growing pains. If the tech you're selling isn't a small incremental improvement on what the customer already has, they'll be glad you stuck around.
Imagine you're selling them advanced battery technology. The customer will do better if you help him through setting up his factory. If he goes it solo there will be way more explosions per capita than if you stay.
Not to mention you'll want to stay on the foreign planet for a while to investigate what they have that you can bring back to your planet. You'll need some training before you can adequately sell what they have.
And, of course, you can be carrying psychoactive spices, gold, nutmeg, or tulips in the hold of your ship as "ballast"... ;D