Five friends and I did a weekend in Dublin for one of my birthdays and we had such a great time we went back and did it all again the following year!
Another great one was my friend's 40th. We went to WildTracks near Newmarket and did the zip-wire, quad biking, go-carting, off-roading in Land Rovers... there are other activities too and you can also drive a tank by prior arrangement. It was soooo much fun! I'm sure there must be plenty of other similar activity places around the country if Newmarket is a problem..
Whatever you decide to do, I hope you have a great birthday!