What a fun travel adventure! Salem has a lot of historic sites to explore. First I recommend ordering a free Travel Salem guide, which will have loads of helpful information. In the meantime, here are some web links you can use to start your planning.
The Salem state capitol building: This is a must-visit, and if you’re not afraid of heights and don’t mind climbing a flight of stairs, you should go all the way up to the balcony on the top of the capitol tower and visit the Golden Pioneer statue. (Plus enjoy gorgeous views out over the capitol city.) Whether you do the tower tour or just the standard building tour, it’s a beautiful building and an interesting tour.
Willamette Heritage Center: This historic site in the center of Salem lets you tour 14 different historic structures, including the oldest remaining wooden frame houses in the Pacific Northwest. There’s a lot to see and do at this one location. I don’t know what dates in May you’ll be traveling, but if you happen to be there on May 12, it’s their annual “Sheep to Shawl Festival” which is a fun demonstration of everything of the entire fiber production process, from sheep-shearing to old-fashioned spinning and lace-making.
In addition, the very day I received your message, an article about historic sites to visit in Salem happened to pop up in my Facebook feed!