For a period of about ten years I bought one new instrument each year. Not a new guitar. A completely new instrument. And as a rule I would learn it in it's native tuning. Much easier to think and sound idiomatically that way.
I think it started with a mandolin. Not a guitar. Not even close. A guitar is largely tuned in fourths. A mandolin is 4 pairs of strings tuned in fifths. Like this one... Fender FM-52E Mandolin, Sunburst
And so it was on. Here are some other stringed instruments I've picked up. Not in this order.
A Hawaiian Lap Steel Guitar - Fender FS-52 Lap Steel
Then I got a five-string banjo. It's tuned like... oh, never mind, let's not go there right now. Not as nice as this one... Deering Goodtime 5-String Banjo
Then a baritone electric. Tuned down a fourth from standard tuning
An auto harp - kind of like this one... Oscar Schmidt 21 Chord Autoharp
Then a uke. - Mine's the flea.
Here's a nice one... OXK Ukulele
An oud. - Similar to... Egyptian Deluxe Oud w/ Soft Case & CD & Oud Pick
A bajo sexto. It's a mexican instrument that is kind of a cross between a baritone and a 12-string. Something like this one... Oscar Schmidt OH52SE Acoustic Electric Guitar, Bajo Sexto
An electric sitar. This Jerry Jones one.
Just added this Gretsch round neck resonator guitar to the collection.
Check out Lark in the Morning for a great source of cheap instruments you might only use once a year.