What plants can you plant close together?

I don't know if it is "correct" but I have 2 watermelon plants growing with bachelors button and I have pumpkins growing with my sunflowers.

these are all 'hungry' crops so could go in the same bed, just allow plenty of room around the squash...........steve

You could take a peep at my blog titled "companion planting" That may give you some more ideas.

Are you thinking of the American Indians' 'three sisters' growing as per Carole Klein in her vegetable series?
The basic principle is to plant the three sisters: sweetcorn, squash and beans all in the same hole.
The theory is that the corn makes a support for the (runner) beans, the squash (courgette or cucumber) helps to suppress weeds by providing a ground cover something like a living mulch and the beans are a nitrogen fixer, improving the soil for both the other crops.

Lora, About the most difficult plant you mentioned is the watermelon, they need LOTS of space (even more than pumpkins) and are suseptible to fungal diseases. Most pros who grow them say to do so separately to avoid spreading any fungus.