Obscuring illusions don't work so well, actually, because they force everyone exposed to them to 'interact' with them (by trying to see through them), thus allowing them a Will save. A great way to blow your cover, really.
In addition, a lot of the suggestions here only work at the high end of illusion, when you can duplicate sound and texture, too....for just sight, it's trickier, because you have to think of things that would also be silent, normally...a much smaller list than you may think....if a pit suddenly manifests, people are going to expect the sound of rocks falling, or grass breaking. If a wall slides into place, where is the sound of it sliding? You can't pull a sword out of nowhere, because most spells make noise when they're cast. You also have to be careful because if they see you casting a spell, you have already aroused their suspicion...
You can do subtle things, though. Just not things that are very effective in direct combat....a few of my favorites....
* The Bar of Gold: Offer to pay an extravagant amount for something, walk into the back room, cast the spell, and walk back out with as many large hunks of metal as you can carry. This works well for other items, too, as long as you have some place to sneak off to. You could reasonably create almost any physical object, though large, blocky things that wouldn't make noise by themselves (furniture, large weapons, things you don't have to put down because they won't make a 'thunk' noise). Heck, if you've got a cart in that back room, use your illusion to make a pile of things. Just make sure to keep concentrating until you've got a head start on whoever you've just stiffed...
* The Bait-And-Switch: This trick requires you to get pretty well ahead of any opponent chasing you, or to know in advance that they are coming. You can cast the illusion of a wall over a doorway or back alley, the illusion of a pit in the ground (that didn't just suddenly appear), perhaps dark and forboding shadows, or horrific skeletal visions. In this way, you can get people to go away from areas you don't want them nosing around in -- such as areas you are hiding in...or get them to go into certain areas (an illusiary sign saying something like "FREE TREASURE --->" can be effective, or an illusary image of your dead body).
* The Codpiece: Useful for dandying yourself or a friend up, have them wear only a few clothes, and then illusion up some resplendant robes around them. If you don't like them, just pull an Emperor's New Clothes at the banquet, and stop concentrating -- one nekkid nobleman, at your service. As an alternative, you could make yourself look dirty or repulsive, too...be careful that you can only add, not actually alter. You can make an illusiary wart or seven, but you couldn't take away the scar you have (though you could maybe magic up a patch of hair over it).
* The Light: Light is totally silent, and thus you can illusion up some light that looks like a torch, or a candle, or whatever. Note that it doesn't actually provide illumination -- it just looks like it does. However, this can be used to make it look like there are people lurking in the shadows, holding torches or candles. If you're among undead, you can probably take more liberties, making them into ghosts or other incorporeal creatures, which are also silent (unless they don't want to be).
* The Better Target: A slight variation on the bar of gold, this just illusions up something better for the beast to attack. Note that this can be quite difficult -- most creatures make noise -- but magicing up a corpse, or a broken wagon overflowing with gold, or a field of sleeping prey can be done without much of a problem. By the time the creature's investigating it, you're hidden behind the illusion of a bush.
Anyhoo, they obviously get more useful as you get to add more senses (sound alone makes for a LOT more versatility), but those should get you started.
