Switching between modded and vanilla (no mods)
To play the game vanilla (with no mods) after you have installed mods, you have several options:
1) You can launch the game directly by running StardewValley.exe rather than launching from Steam/SMAPI. You won't get achievements this way.
2) You can remove the Steam launch options and then launch from Steam, but you have to remember to put back the launch options to play modded again. This way you'll still get achievements.
3) You can create a modgroup with an empty Mods folder, and launch with that empty modgroup (you can name the folder
Mods (empty hehe), as an example). That way, you can still launch from Steam without changing launch options, and still get achievements. For more details on how to create modgroups, check out this section of the getting started guide.