Some plant suggestions might be easier to provide if we knew the cold and heat extremes or your region.

But without knowing anything about the temperature ranges and solar exposure, I would say some type of emphemeral (i.e spring blooming) bulb plant would work for you like a daffodil or hyacinth.

It comes in like a lion and leaves like a lamb ready for the next cast of players to take the stage!

South Finland?

Alyssum and lobelia will grow and spread quickly from seed. start it indoors and plant out as soon as you can.

Spring bulbs, as already mentioned, can be planted this fall for next year.

Suggest you repost your question on the Perennials Forum where you should receive some helpful suggestions. I am not certain of your growing zone but Bergenia and primoses might be good filler plants to consider. Also, almost unknown is the fact that peony foliage can be cut back to the ground with no harm to next year's growth about the end of July. I have often used this method in my perennial garden designs to fill in blank spaces while waiting for fall blooming perennials to shape up.