I'm hoping to share some DIY and home improvement tips here as I come across them. This is a good one for a smelly drain in my old house.
We noticed that when it's cold, a bad smell comes from the bathroom area after a shower. At first I thought it might be leaky pipes (a scary thought since there's not much in the way of crawl space beneath the bathroom - more like squeeze space). I did some investigating online and found that it's probably old water getting stuck in the trap.
I started pouring a couple of cups of vinegar down the drain after each shower. No more bad smell!
Of course it would be ideal to inspect the area and remove any clogs that are found, but this fix is working for me in the meantime. (We're planning a big bathroom renovation sometime down the road, so this isn't our idea of a permanent fix.)
You can get huge jugs of Vinegar at BJ's for less than $4. A jug lasts more than a month when used for this process, and I don't have to do it in the summer, so it's a great inexpensive fix for our smelly shower.