Hi @BlueLine ,
Your first backpacking trip is always so exciting! I recommend looking around the I-90 Snoqualmie area or just outside of Greenwater, WA. A lot of hikes in those areas would be fun for an overnight trip! Usually these areas require a praking permit (Discover Pass or Northwest Forest Pass), but do not have specific permits to reserve overnight camping.
I realize your original post was last month. Sorry if this is too late!