Recently I've been stringing my 25yd sprints into 100yd swims. First 25 is all out, then use the remaining 75yds as a recovery swim. Will do sets of 3s on a 2:00 interval, rest 1 minute, then start the next set. I'll also change things up by doing the 1st 25 no breath, hit the turn, and try 3 sdks + 2 strokes before my 1st breath.
As an aside, I find sprint training tougher than workout swimming. It's shorter, but hurts much more (at least for me).