Okay, here we could have been more prepared. Make sure you check out timetables and price listings for the cable cars so you don't end up getting to the top and then scrambling to take the last cable car back down (of course, if you ski or snowboard down, this is not a problem for you).
We made it up to Kleine Matterhorn, but you have to take an elevator up to Glacier Paradise to get the great view and walk inside the glacier, but we just missed the last trip up. Apparently it's only open until 3:45pm (we should have looked at these details beforehand at http://bergbahnen.zermatt.ch/e/peaks/kleinmatterhorn.html). It was sad. Lesson learned? Don't spend so much time in the grocery store, eating, using the toilets, or taking pictures UNTIL you've at least gotten to where you're going. Also, I think this is where spending the night in Zermatt or at least in a town a bit closer (we came from Bern, and that takes 4 hours) comes in handy so that you can get up in the cable cars in the morning and have more time to wander around, take pictures, and not miss the last cable cars.
Weather: I have heard that the weather changes rapidly in the mountains, and often people get to Zermatt and can't even SEE the Matterhorn (this is actually how it was most of the time I was there over Christmas), but we were lucky. On the Zermatt website (www.zermatt.ch), you can check out the view through the webcam, but of course, if you have to make plans and book hotels, you will just have to wing it. We were fortunate enough to have AMAZING weather.