You must try Triund trek...It's a gorgeous trekking destination, most famous amongst beginners owing to its easy gradient and a short distance from the base camp. Despite of its short and easy hike, it still provides with a dazzling mountain experience and all the fun and excitement that one looks forward to in a good trekking expedition and it's the best if you are looking for a day trek.. The best part about Triund trek is it can easily be done solo due to its well defined route. Trek to triund starts from Mcleod Ganj with total trek distance of 8-9 km one way but you can also ride till Gallu Devi temple (1st check-point of trek) in a shared cab/taxi/ auto but they charge more due to bad road condition. You can save 1- 2 hrs by taking a cab/auto ride till Gallu Devi temple and then starting the trek.
If you want a one day trek in himachal a then h go for triund trek..
If u want to go for a one day trek in Uttarakhand then go for deoria tal trek.
Wherever u choose just make sure u have a rain coat, umbrella, torch and a good non slippery sports shoes with you in the monsoon season.
Enjoy the trek