I just went a different way or tried something different with each section - which I think was what they may have intended in the design, because I did have a lot of fun throughout Halo. Those repetative sections were familier to you in that you knew the approximate layout of what was ahead and so could try things you couldn't (without having died at least once) in the Half Life series style fancy tunnel.
I feel the repetition allowed you to plan ahead rather than just react quickly, which was cool. It mixed things up just enough with different enemies, placements and obstacles.

(Well, the round room things more than the library - the library did get quite repetative, mainly because there's not a many different tactics you can use against the Flood the way you can the Covenant)
My main gripe about the Halo series (apart from me not liking consoles) was how they handled the Flood sound effects. They got the look right - there's something quite unnerving about the way the elite-forms heads just flop around and stuff, but they totally kill off the potential scare factor by making the things gurgle and burp all the time. :\
With some propper undead screams, roars or at least strangled moans those things could have been pants-crappingly scary in a few places.