During a UG physics degree , what other certificate/diploma courses can i simultaneously pursue which would be of use in a physics career?
This answer is from the opposite end; I studied engineering and took some additional physics courses on the side. Through those, I developed some idea of what undergraduate students in both disciplines might typically lack. 

Physics is incredibly broad, and you may be able to choose meaningful courses only when you have some idea of what field you want to specialised in, or what kind of job you would like.

Nevertheless, one skill that will not go waste is being able to write good, efficient programs. All you need is a computer and an internet connection, there are fantastic courses online today. A lot of open course ware exists with various certification options. What's most exciting is that many programming courses are tailored towards research and data science; both of which you are certain to use at some point. I don't think ASE encourages recommendation of specific courses, so I will refrain from that; but if you look around you will certainly find interesting courses.

In addition, if you are looking at applied physics fields, I would recommend two courses from the engineering side- basic electronics and strength of materials. These two will be useful when you want to design your own experimental setup, and having prior knowledge will put you ahead of the typical undergrad student.

Finally, statistics and probability courses will certainly be part of your regular course. If you, however, feel that these aren't rigorous, you may consider doing some advanced courses. It could be very useful when dealing with large sets of experimental data.