I have a bed for tomatos. Very early in the season, a month before last frost, I transplant broccoli starts into it. When it's warm enough I put the tomato seedlings behind them. By the time the tomato plants need more room, the broccoli is done and I pull it.
In the meantime, I put in marigolds, nasturtiums, onions and carrots around the edges. This year I tried borage, which is supposed to be a good companion plant for tomatoes, but in my limited space it quickly got too huge and was crowding everything, so I pulled it out again.
Soon I will start some lettuce in any spaces left, for a fall crop. (Carrots are about ready for harvest) The tomatoes will provide shade for the lettuce while it is still hot. Once the tomatoes are done for the season I will put plastic over the lettuce and hope to keep it going for awhile. By this means, I will have things growing/ producing in that bed, from early March well into November. Pretty good for zone 6!