Question: Can I sunbathe on Shabbos?
Answer: The Shulchan Aruch (OC 303:25) writes that the melacha of tzoveia, dyeing, applies to colouring one’s body. R’ Yitzchak Yaakov Weiss (Minchas Yitzchak 5:32:2) writes, therefore, that one mustn’t sunbathe on Shabbos, whether for medical reasons or just because they would like a tan. Additionally, he argues, that when it is exceedingly hot, sunbathing can be a painful experience and so must be avoided on Shabbos (See Rambam, Shabbos 21:29). Lastly, sunbathing is normally preceded by rubbing sun-cream into oneself, and often ointments afterwards which is prohibited on Shabbos (See Chelkas Yaakov 4:17:1).
R’ Binyamin Zilber (Az Nidberu 2:30) writes that while one can’t sunbathe for health reasons, one may do so on one’s own balcony for relaxation.
R’ Moshe Feinstein (Igros Moshe OC 3:45) writes that while photochromic lenses change colour in the sun, there is no issue of tzoveia as it is temporary and changes right back. R’ Yehoshua Neuwirth (Shemiras Shabbos Kehilchasa 18:n70) compares this to sunbathing, which leaves a temporary tan, implying that it isn’t a problem.
R’ Pesach Eliyahu Falk (Machazeh Eliyahu 1:65:6:24) writes that while purposely sunbathing may be problematic, one can certainly relax outside in the sun if their intention isn’t to get a tan.
In conclusion, one may relax outdoors on Shabbos if they aren’t purposely trying to tan providing they won’t burn.