Suisun City Marina is one of my favorite destinations. It has a very large guest dock, the city runs several events on the waterfront in season (art fairs, concerts, etc,)there are several blocks' worth of historical buildings, and many restaurants worthy of a visit by themselves. My favorite is a Greek grill -- I believe it's called the Athenian Grill -- but there are others.
The boat ride there is worth the trip by itself -- very scenic and a little challenging as well. From Martinez, go up the river and past the Reserve Fleet, and follow the markers to enter Montezuma Slough. Then take Hunter Cut into Suisun Slough, and follow the slough to the marina. Unless you have a sailboat with deep keel, this can be done at low tide as well, as long as you avoid the lower Suisun Slough (south of Hunter Cut)which is really, really shallow. Watch the channel, watch your depth, don't go without a chart (GPS navigation would be better), and watch your depth.
Realistically, the main peril in the slough, in my opinion, is not grounding, but collision with speedboats coming at full tilt around the several blind curves. Also worth mentioning is that there are several privately maintained markers on the slough, showing shallow areas. In my experience, they tell the truth...