There are any number of perennials, both old fashioned and modern, that will always look good in a mixed bed, either by colour, size or shape, but I suspect are usually bought in from the nursery or garden centre. Nothing wrong with that, but have a go at sowing your own from seed.  There are some that could grow and bloom later in the first year, such as lupin, delphinium, pinks – if they don’t you’ll have stock ready for the following year. So have a go now with moist seed compost and sow very thinly with only a slim layer of compost on top and cover with opaque polythene.  Keep in a semi-light place. Do check regularly and, as soon as green shoots appear, move into full light. Use caution with watering as they will be better on the dry side than too wet. Too high a temperature, however, could result in thin, weedy seedlings and what you want are short, bushy ones which, at about ½” high and need to be thinned to give each seedling room to grow strong, healthy roots and upper greenery.