Which String Instrument Would You Recommend After Guitar?

Hi everyone,

I've always enjoyed listening to music, but recently I've been thinking about learning a string instrument beyond the guitar.

I work in medical billing At Avenue Billing Services, so most of my day is spent at a desk. I'm looking for a hobby that's relaxing, rewarding, and gives me a break from work. I've been considering instruments like the mandolin, ukulele, banjo, or even the violin, but I'm not sure which would be the best choice for a beginner.

I'd love to hear your opinions.

Which string instrument would you recommend for someone starting from scratch?
Which one has the shortest learning curve?
Is there an instrument that's especially enjoyable for playing folk or acoustic music?
What do you wish you had known before learning your first non-guitar string instrument?

I'm looking forward to hearing your experiences and recommendations. Thanks in advance!