One focus area can be to provide a nostalgic gift to them. The gift should bring back some good old memories. You can do some basic research about those times and choose a gift based on that. The only thing to take care is not to buy a gift which they already own. Since they are born in those times, they may already have some of those items. Another option can be something funny. This can be based on the age or their period of birth, which will help lighten the mood. There is also another interesting option where you can gift them an experience like driving a sports car, night life or some adventure sports. This will certainly add some excitement to their birthday making it memorable.
Many people find some gifts to be useless as it becomes a redundant item. You can think of some gift which will be really useful to them like a display art or a cuckoo clock. If you can do some research on their day of birth, you may be able to find some important milestone in history that happened on the same day. This will bring in a feel-good factor. Buying exotic alcohol is a nice option. While you may not want to buy the normal supermarket alcohol, a bottle of aged premium malt whiskey or a champagne set will always be a good choice for the ideal birthday gift. Weigh these options and take time to choose that unique gift, and make the 40th birthday special for your near and dear one special. You will find a wonderful way to make the day extra special.