My main instrument is the guitar but here are some (if not all) of the other instruments I've been playing. Many are stringed but recently I got a recorder which has been more challenging than I originally thought. It felt good when I was talking with a friend who has a Native Indian flute and expressed he was having the same issues.
There is no particular order to the list.
Non-Pedal Steel Guitar
Lap Steel Acoustic (Weissenborn Style)
Cumbus (pronounced joom-bush)
The way I look at other stringed instruments is through the tunings. Once you understand the tunings and can begin building various chord shapes and scale diagrams its the matter of putting it to practice. Different instruments tuned to similar tunings makes it easier to go from one to the other.
IMO, the mandolin is an excellent choice if you are interested in trying out another instrument while staying within the plucked string family.
I played recorder when I was a teenager - and that's how I learned the basics of music thoery and notation by the way. Today, besides my guitars, I sometimes play ukulele, and I've been back to harmonica. I had some notions, mostly learned on my own, on harmonica when I was younger, and now I've bought a method to learn it more seriously. But guitar play takes most of my musical time though.
About other fretted instruments I'd like to play, I'd go for mandolin and banjo. But for the moment I have neither the time to practice new instrument, nor the budget to get them. But anyway, it's always enriching to play various instruments and styles of music. Guitar is very much this way through the large number of types of guitars that exist.