Al. Nieto looks obvious to do but I've not been close to it. I went part way up the Valle Frances last year and it looked fairly straightforward to get up close beneath the Fortaleza. Not sure there are any scrambling routes on the Fort itself. The original route has only been repeated once or twice, one of those in 8hrs or so by Charlie Fowler, but it has an 'ice hose' water-ice pitch down low, or at least mid-height. There's another thing up there somewhere - the Shark's Fin?? - that a friend climbed part way up years ago without too much trouble.
Around up the Ascencio, up past Japanese Camp, you can walk/scramble up to the notch between the N and C tower, as if to start the Monzino route on the N tower, but there have been rockfall accidents here, some fatal.