It is finally safe to plant out half-hardy summer bedding and patio plants - geranium, stock, petunia, marigold, nemesia, osteospermum, verbena, bacopa, argyranthemum, to mention a few - in all but the very coldest parts of the country. Be choosy about the quality of plant you buy: look for strong, healthy plants with many buds rather than those that are in the full flush of flower. Buy from a reputable outlet, where the plants are likely to have been properly looked after, and check that the compost is nicely moist and not dried out. Ask if the plants have been "hardened off" - acclimatised to outdoor conditions. If not, leave the plants outside during the day, and bring them in at night for about a week.
In the garden, plant them in naturalistic pools and drifts, or use them to fill gaps in the border. If planting in containers, run some slow-release fertiliser or composted chicken manure through the compost and add water-retaining gel crystals if you're less than disciplined about watering. Deadhead regularly to prolong flowering.