[Gordon Currie]

You've got some good ideas so far - I totally agree with keep it simple and stick to the melody as much as you can.

Another technique I used to employ when I was doing the solo thing - especially on a vocal tune where there wasn't a specific soloing section - is to alternate between strumming or arpeggiating and soloing. I might play 3-4 notes and then strum a couple of beats, then more notes etc. This would give the illusion of something going on without completely losing the momentum of the chordal backing. A little like juggling.

Some songs you can actually do a call and response between (e.g.) 1 bar solo and 1 bar chords - that can work well. Other songs start sounding empty more quickly so you might want to switch back and forth every two beats or so.

If you incorporate double stops as well that spices things up considerably.

Even though I don't play solo anymore, I still use this general technique often.