Sometimes I focus mainly on one sense, other times it's a crossover. Here are some of my successes:
There are lots of activities for touch, but my classroom favourite is my magic bag. When I'm introducing a new vocabulary word, sometimes I'll bring a representation of the word and "hide" it in a green velvet bag I bought at Ikea. I pass the bag around, and they try to guess what's inside by how it feels.
I like to bring out play dough, sometimes with a theme, to play with at least weekly.
I hide objects (sliced fruit, cotton with a few drops of essential oil, pine needles, etc.) in a container covered in parchment paper with a few holes poked into it. They guess what the smell is.
To look more closely at plants (but you could do it with other objects), I pop them into a jar or bottle of water. It's more awe-inspiring than simply passing around a leaf, not to mention I go through fewer samples.
We play games like I Spy, and use Where's Waldo-type posters to spot different things. We have colour theme days, where we try to spot as many objects of the day's colour as possible, or we look for examples of opposites (right now big and small).
They love nature sound effects! This is easy to provide with a CD or even from my phone. If we're talking about a certain animal, I'll ask what noise they think it makes. Then we listen to the real thing, or I'll play a different animal (for example, a dog when it should be a cat), and they "correct" me.
We play whispering games during circle time (a very simple Broken Telephone game), and repeating games (where we drill vocabulary using different tones - the favourite is "mouse voice"), or I speak really quietly so they really have to concentrate. Speaking quietly has the bonus effect of the students modulating their voices to match my level. A nice, easy way to have some quiet time!
This one is trickier to tackle in the classroom - dietary restrictions and severe allergies keep me from passing around food samples - but we do eat together twice a day, during recess and at lunch. We talk about different tastes and preferences then, as well as discussing foods we've tried at home. Right now we're focusing on the basics (sweet, savoury, bitter...).
Special sensory activities are fun to prepare and fun to deliver. The students are motivated when they know we've got something planned for later in the day or week, and so am I!