What can I get my niece's husband for his birthday? He's in his mid 30's....?

Depending on how much you are willing to spend, you could get him a watch (even a Harley watch!) for anywhere from $50 to $200. Or perhaps a GPS (under $100 to $400 or so) for his motorcycle, people who ride motorcycles usually like to go for long rides and try new paths, therefore getting lost is often part of the adventure.