You could plant some bulbs that open early in the year, which I always experience as an invitation to enter the garden when there still isn’t that much to do. Add some other flowering plants, perennials or self-sowing annuals, that begin flowering from May on. I personally like calendula, but one large patch of it might become boring after a while. Nevertheless, an easy, undemanding and beautiful plant.
Perhaps a note on what you should not do: plant an evergreen low-growth bush-like cover, of a type that is often used to reduce garden maintenance for those that like to have something green in their garden but don’t want/can’t involve themselves with gardening. They are often a hiding place for slugs, which I assume you don’t want in your allotment.