This is my normal go-to raita, not quite sure why I do this so often rather than the tomato one. You could always put tomato and cucumber too, of course.
Half a long cucumber (partly remove the peel, in strips, and slice finely)
1/2 teaspoon mustard powder
1/2 teaspoon salt
1 green chilli, slit along its length
Small handful coriander leaves, torn up
1 standard pot of yogurt (1/2 litre?, stirred up so no lumps)
Mix all ingredients into a bowl, stirring well. Leave it standing for a while to let tastes blend in. Before serving, remove the chilli. Garnish with coriander leaves.