If someone has a green horse they want to take on a trail Lord's Hill between Snohomish and Monroe is ideal. Nice, easy trails and a good parking area. The hill up to the trailhead is kind of steep but we made it and I consider our truck gutless.
My favorite is to go over to Cle Elm, there is a horse camp with corrals and camp spots for the 2 leggeds. I haven't been there for 3-4 years now but it was in fairly good shape last time we were there. The trails run different degrees of difficult. You can ride to the lake, it's not bad or if you want to go for 22 miles you can ride up to Wapato. It's gorgeous, some up and downs and a small stretch of rock to get over. You can see the wild violets in May, but might also run into snow. There's lots of trails in this area, one of my favorite places.
Haney Meadows is another fun one if you can make it up the switchback with your truck and trailer. That's a test right there! I'd love to go down to Capitol Forest, I'll have to put that on the list!