I didn't know what to use for the postcard, so I used an old render with colors that I thought fit the scene.

Thanks (about the starfield trick) I really like the look of the table cloth. Is that a procedural material ? Difficult to imagine what the liquid might be. Maybe a different color would help a little. I like the idea of a background really blurred. It brings up the main subject of the scene a lot more.

Really like the cloth and the blurred background. The ice looks good to.

Really like the colors you chose - and that tablecloth is fantastic!

I thought the liquid looked off. You see I'm green color blind. I thought I'd started with a color map called Valentia orange. Perhaps, the lighting has effected the result. I'll redo the color there.

The table cloth material is procedural with two nodes of 'varying blocks'. Further, I duplicated the material into a mixture and reversed the bump polarity on the duplicated material.

Increased the lights on the cubes.
Changed the color map and murky colors for liquid.
Lowered the green influence in sky, backgrounds (hidden), and ambient light.
Lowered reflection of liquid from 40% to 10%.
Made the straw semi-transparent.

Cool picture. I like the pencil and the straw idea. I am not a fan of the backround. The tablecloth has a pattern that looks like a shadow that I don't like. JMHO. I like the picture on the postcard. Good work.