Deadheads on geraniums or other balcony plants not only look ugly, they can also spread botrytis cinerea or other diseases. So make sure to remove them on a regular basis.
Lots of tomato varieties such as trusses of tomatoes or cocktail tomatoes grow well in tubs on the balcony. The plants should be placed in a sunny but rain protected position. A peat-free soil specially designed for the needs of tomatoes and vegetables makes a suitable planting substrate.
Roses in tubs easily succumb to a variety of fungal diseases such as black spot. Spray your plants before infestation with a product against fungal diseases on ornamental plants. Ask at your garden centre for a suitable product.