By physical activities I mean any kind of task, work, sport, you have to use your body. Not always have the time to do some task or even the possibility (need some equipment, not great situation). I try to imagine myself doing what I intend to practice. Example: some katas (karate movements), sign language, arts&crafts. Is like I imagine how will it be so when I can do it, I can do it better. Do I need to practce the real movements to improve or by thinking I have enough? 
Can you practice and improve physical activities only by thinking?
learning;performance;motor;imagery
My general understanding is that thinking about executing the motor tasks in your mind can improve skill. It's not as good as actually practicing the task. I'm not sure exactly how conclusive the literature is on what moderates the effectiveness of mental practice. However, presumably, the nature of the task, the nature of the mental activities performed, your current skill level, and so on, should be relevant.Perhaps check out Driskell et al (1994) for a review and a meta-analysis. It has a more nuanced treatment. Specifically, they define the relevant act as mental practice:Mental practice is the symbolic, covert, mental rehearsal of a task in  the absence of actual, overt, physical rehearsal.So, it's not just any kind of thinking.ReferencesPascual-Leone, A., Nguyet, D., Cohen, L. G., Brasil-Neto, J. P., Cammarota, A., & Hallett, M. (1995). Modulation of muscle responses evoked by transcranial magnetic stimulation during the acquisition of new fine motor skills. Journal of neurophysiology, 74(3), 1037-1045. https://www.researchgate.net/profile/Aidan_Moran/publication/228118007_Does_Mental_Practice_Enhance_Performance/links/00b49527e74ea53a69000000.pdf