Talking about easy topics, there are no easy topics related to botany. At least, there is no easy topic that would be available for a thesis. Before starting your thesis work, make sure that you are doing it in an area in which you have a great level of understanding. Otherwise, you might get stuck with it for a year or two.
Apart from that, make sure to conduct a good level of investigation. There are various good articles available on google that tell you how to choose a thesis topic. Go through these articles, and you will be good to go.