Sounds like you had a good time and it's great you got out on the Naches!
If you have more time the trail will drop you into the Naches Basin off Hwy 410 in eastern Washington. From there you can climb up and over Manastash Ridge which is a 115 sq mile piece of territory criss-crossed by 4x4 and motorbike trails. LOTS of stuff to keep you entertained for days. Great place to go camping, too. And you're stock Jeep should be able to handle most of it with ease. You can exit from the Manastash Ridge area to the north and hit I-90 near Cle Elum or Ellensburg.
An alternative are the trails north of Cle Elum and Roslyn. We just did the Fortune Creek trail this Saturday with a fellow in his Landrover LR3. They're fun but probably a smidge easier than the Naches. Again, good places to work in some camping, too.
Someone mentioned Libery, that's another good area for some intro wheel'n trails and there's some tougher stuff as well.
And there are a number of places / trails at Reiter you can have fun on as well. I might recommend Reiter for mid-to-late October. There are a number of big maple trees in the lower forest area that have great fall color about that time of year.
If you want to step it up a bit check out Evan's Creek, the harder trails at Rieter, and Walker Valley.
Here are some trip reports / photos from some recent runs:
North Cle Elum (Fortune Creek Trail) - Trip Report
North Cle Elum (Fortune Creek Trail) - Photos
Naches - Manastash 4-day Expedition - Trip Report
Naches - Manastash 4-day Expedition - Photos
Some pictures to get you thinking . . .
Landrover on the Fortune Creek trail in the North Cle Elum area.
Hawkins Mountain trail at the end of the Fortune
Creek trail north of Cle Elum.
Fortune Creek Trail (lower portion) north of Cle Elum.
Manastash Ridge area just east of Moonrocks.
Tipover Trail in the Manastash Area.
Reiter Trails under the power lines.