If you have a tree in your lawn you can leave a border of grass to grow around it. Plant some flower bulbs such as daffodils (native ones, not double-petalled) and bluebells. Also try crocuses, ...
If you haven't got a lot of space, you can grow wildflowers in a container. You'll be able to enjoy a burst of colourful and interesting flowers while watching visiting bees, butterflies and other ...