I used to teach guitar, and one of the trick I used to get my students to improvise was this:
I'd give them two notes, say the root and the flatted third (I guess I'm teaching the blues here) and I'd play a simple 1/4/5 progression nice and slow. I'd say, OK, play anything you want, but you can only use those two notes. So what does the student do? They're stuck with putting two notes together in as many different ways as possible. Can you play a triplet? How about two sixteenths and an eigth note? How many patterns can you make with two notes and every possible rythm combination? Do any of them sound familiar.
When you master that, try adding the fourth. Now you've got three notes...
Back when I was being a professional musician I had a couple of rules I followed: If you can't think of anything clever to do - reach into your bag of tricks. Of course that means you've got to have a bag of tricks. If you remember how to play two notes like they're a solo, you're off to a good start. The other rule was, if you make a mistake, do it three times and then correct it (they say jazz is riffing on your mistakes). For example, you hit an F# instead of a G and it sounds so sour it makes you squirm. Hit it again and squirm some more, hit it again and squirm some more, hit it again and
slide up to that G. Try it, it works great, and if you don't tell anyone it was a mistake, you're Miles Davis...
Oh yeah, that was rule 3 - you never let on it was a mistake. In music, you're only as good as the people you can fool. I mean if my ears aren't good enough to tell you're a little out of tune or a little late hitting a note, and you don't tell me, I won't know. If you're playing for my son, (who's a music producer in Maine), he'll grab the guitar out of your hands and tune it for you; his ears are better than yours will ever be. He'll tell you where that note should have landed, and if you can't do it, he'll edit what you play to make that note land where it should have... 9 times out of ten, you'll be the best musician in the room, and Mike lives in Maine so he won't bother you...