South sister is a blast to hike if you don't have any major experience. you will only cross a little bit of snow this time of year and the trails are marked well.
North and Middle Sister are much more technical (especially North).
From the start of S. Sister Climbing trail off cascade lakes highway its about 14 miles round trip and makes a good day trip if you go lightweight. It also connects to the Pacific Coast Trial (the biggest network of trails on the west coast) if you want to make it a multiday trip.
Just pay attention to the weather if you plan on going to any summit... it can get real bad real quick. just my