Whether they have been overdone or not, I am still in love with garlands. While material varies, and there are too many DIY garland projects online to count, I really like this reusable garland from Fellow, Fellow. You could swap out the yarn with any colour you choose, and store it for the next birthday or even for an impromptu message.
A few weeks ago I picked up these letter stencils with another project in mind, but for a quick birthday party message I strung together a few stencils and fastened it to the wall with washi tape. A super easy project that took less than two minutes from start to finish. You could punch holes on the top corners of the stencils to weave them together but I like the look of the more haphazard arrangement.