I agree with what people are saying. The coast is a pretty long drive from either Mt. Hood or Mt. St. Helens. Mt. St. Helens is a nice day trip and very cool, but I don't know if it's worth driving all the way from the coast. I don't know if Mt. Hood is really anything different from other mountains you can see other places. If you only have one day in Oregon, I'm not sure if I recommend that. We also love Multnomah Falls and the gorge, but again, it's a lot of driving if you only have one day. Depends on what you really want to see and do though and if you like driving, it's a beautiful drive this time of year.
If you stick to the coast, my favorite coast towns are Astoria, Seaside and Cannon Beach and we love Moe's if you want a good bowl of chowder!