As for Sand Mtn, weekends are wild mid week mild. It is just a huge 700' dune in the middle of nowhere. Takes some horsepower to enjoy it otherwise, you'll just be tooling around the bottom.
Make sure you pack all you need as Fallon is 20 miles back. There's an old Pony Express stop on the way into the recreation area and some trails to explore further north of the mtn.
Just remember Google Earth is your friend. Many N. Nevada trails show up very nicely on Google Earth and with Earthbridge or GooPS and a laptop you can map a trail run in advance and take it all with you...that is if you're not one of those that likes to get lost in the mountains