Lots of shrubs e.g Hebe, Hypericum, Fuchsia magellanica could be kept to 2-3 feet high, but perennials that don't die back would be more self-limiting.
Some Nepetas eg N. faassenii are the right size, Perovskia "Blue Spire", Lavenders, Caryopteris. I've just noticed they're all blue/grey.

thank you for your suggestions

You're welcome! There are bound to be others. I thought of small Euonymus as well, e.g Emerald & Gold, Harlequin can be kept relatively small.