Would you consider a clematis? There is the "boulevard collection", which are suitable for pots, and quite a few of them flower in May. (You can buy online and the company I have used seems very reliable).
I'd plant some bulbs. Tulips would be ideal - colourful, cheap and hard to get wrong (unless you plant then upside down). ! Plenty of choice too. RV Rogers or Peter Nyssen are two reputable suppliers. Good luck.
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