Laique,
An 'easy topic' will be the one, which is related to the structural course you like most to study and learn. However, actual research you will have to carry out, may or may not be easy in that sense, as it will depend upon the 'research type' (as explained below) you select for your topic.
Research topics at MS level are of generally two types. The first requiring analytical research, and the second involving experimental work, to be carried in a laboratory through physical testing of specimens.
First type of research could be very inexpensive in case, firstly it can be carried out using existing resources and secondly if the availability of relevant data could be made economically. However, selection of a topic for this kind of research and actually getting through this kind of research might not be easy.
Second type of i.e., experimental research is more common, as it is not only easier to carry out, but also easier to get through with your final exam as well. It may also be comparatively less expensive, if it mostly requires commonly used & locally available material and does not involve extensive use of advanced but costly research materials (like fiber reinforced polymers (FRP), commonly being used in structural rehabilitation & retrofitting related research now-a-days).
Regards.