The next two flowers I'd like to present constitute good "fillers". Neither one is exceptional by itself but within a border, they both add an interesting element and blend well with the showy ones.
Yellow Loosestrife (Lysimachia punctata) is a tall plant, growing 2-3 feet tall. Its flowers grow in elegant flowering spikes. It's great in a perennial border or beside a water feature.
It is in no way related to Purple Loosestrife Lythrum salicaria, an invasive plant that everyone attempted to eradicate in the 90's.
I have 2 areas where Yellow Loosestrife has spread nicely. Above you see it with red Serviceberries but I also like it with the annual bright red Maltese Cross (Lychnis chalcedonica)
Yellow Loosestrife is best left in the border. I have tried cutting it for bouquets but is short lived, lasting only a day or two.
Ladies Mantle Alchemilla mollis is more chartreuse than yellow. Its flowers are quite insignificant but the plant grows into huge large mounds (2 - 2 1/2') with wonderful fuzzy cupped leaves that hold water droplets after a rain.
It blends will into foreground or background and makes plants beside it look fabulous.
If you have never tried either of these plants, please do. I hope you won't be disappointed. Both are very low maintenance and tolerated last summer's heat and drought very well.