That's a good analysis, and the harp and electronic keyboard are both easy instruments to learn. I would add that the guitar also allows for:

• nearly mindless strumming that will still produce many interesting variations on popular chords

To get, e..g, different kinds of E Major sounds on a piano, you have to remember the finger shape for E Major and move it the correct number of spaces up and down the keys. To get that on a guitar, you just move one hand rapidly back and forth, hitting different strings at random. Maybe you occasionally release a finger on your nondominant hand.