A (landmark) birthday doesn't necessarily have to involve lots of other people. Do something that pleases YOU if you want to celebrate it in your own special way - go somewhere new or somewhere from your childhood that you loved, or just stay at home and keep it low key but nice - stuff your face with crisps, sweets, jelly and a caterpillar cake. Or maybe that's just my ideal birthday!
It's YOUR day after all, whether you choose to celebrate it or not.
I went to Cornwall for my 30th birthday for a couple of days with a friend - it was all planned by my own self and I did everything I wanted to do, visited some old sights from childhood holidays and didn't have a "celebration" as such. I just wanted to do something a bit different than my usual sitting at home and it all worked out okay.