As far as SHREK goes, the first did a good job at establishing the characters, settings, and universe, but the second one is one of the greatest sequels ever produced in terms of storytelling, the introduction of new characters, and all points inbetween. The third one was a step down in quality, but if you're looking for a bunch of quick and easy laughs, then you can still enjoy it. The fourth felt like it was ready to end, and I'm glad it did, rather than just keep on going for several more sequels like they were planning to.
As far as ICE AGE goes, the first movie was fun and quirky, but it felt fine as a stand-alone movie. The second one is awful . . . some of the new characters were fun and enjoyable, but the overall movie just falls flat. The third one was okay, but given the timing that it was released when that LAND OF THE LOST remake was as well felt like they were basically just ripping off LOTL. I haven't bothered with any of the others beyond that, and I keep hearing rumors that a sixth movie may be possible.