There you can do some activities like renting bicycles, pedal boats (80 THB per half hour, 150 THB per hour), kayaks and ATV. There is also a space for swimming delimited by buoys. In the rest of the Huay Thung Tao lake, however, it is forbidden to swim. I have not tested yet but it is possible to spend a night here in the camping area (tents provided). Apparently you can also get a massage. In short, there is plenty to d here even though many activities are mostly offered on weekends when many people come from Chiang Mai to spend the day with their family. Notice that during the rainy season, the waters may be less clear for swimming but the setting remains pleasant. And do not forget to go see the giant King Kongs made with rice straw !
I don't think, there are public transport to come to Huay Thung Tao Reservoir from Chiang Mai, so you will have to be transported or hire a songthaew (red car). Whatever the number of passengers, it should cost you around 600-700 THB to get you there, have the driver wait for you several hours and bring you back to the city.