Jesus Christ, seeing a thread asking for "old" UFC events in which UFC 100 counts as old makes me feel fucking old.
Anyway, to cast a wider old school net, my favorite events have always been UFC 6, UFC 31, and UFC 34. Top to bottom awesome fights on all three cards featuring a ton of legendary fighters. For your window, definitely be sure to watch UFC 31 before UFC 34, as the latter features the rematch between Randy Couture and Pedro Rizzo following their epic back-and-forth war from UFC 31. UFC 38 also gets overlooked, but the rematch between Matt Hughes and Carlos Newton - following their iconic first match at UFC 34 - is one of the greatest showcases of ground control and positional dominance courtesy of Hughes.
There's so much awesome stuff from the early Zuffa days, to say nothing of the pre-Zuffa days, but to keep myself from just listing it all, do yourself a favor and check out UFC 31 and UFC 34 if you haven't already seen them.