Mason Lake is a fine spot, but it will be very, very busy on the weekend. If you keep on going into Rainbow or Island it would probably be far less crowded. Talapus and Olallie are too crowded on the weekend, but going over to Pratt would be nice.
The Foss Lakes (Copper, Little Heart, Big Heart, etc.) are always nice. Necklace Valley is great.
Snow Lake sees a lot of people, but it is worth it. You can continue on to the Wildcat Lakes.
Nordrum or Snoqualmie Lakes are great. Or you could go up Dingford Creek and hit Myrtle or Hester.
I haven’t even gotten outside the Alpine Lakes Wilderness! So many great options.