You can have an evergreen in there, like a box ball or a small conifer, for permanent structure, and plant large bulbs deeper than the smaller ones, so that you can get more in the pot. You could get quite a long spring display like this, then when the bulbs die back plant seasonal plants for summer, removing them when they are over and repacing them with autumn and winter flowerers. If you plant pansies, they will flower for months if you deadhead them, for example. Just make sure that the bulbs are planted deeper than usual and top up the pot with compost when they are over. You'll also have to make sure you feed the plants regularly even if you put in slow-release fertiliser at the beginning.