Bath salts are super easy and very popular. Make them more exotic by mixing sea salts, Dead sea salts, and Epsom salts. Cellophane bags run about $5.00 for 100, and you can get some cute ribbon or colourful twist ties to finish it off. There are tons of cute envelopes and boxes on the 'net if you want to go further and make something like that.
Bath bombs are cute and easy as well. Get a basic recipe, a few colours, a few scents, some silicone ice cube molds from somewhere like Ikea or Linens & Things, wait 45 minutes, pop them out and you've got bath bombs! (If you roll the bath bombs in your hands to make small round ones, you don't even need to buy molds!)
Beeswax candles are similarly easy and cute.
Here's a nice tutorial for sushi candles: http://www.wicksandwax.com/sushi.htm
I don't know if you are near a Daiso, but get some sushi take out boxes (18 for $2.00) and some fake grass and you have some lovely candles! (Ignore the part about using a hair dryer. I just rub the wax with my fingers and it gets soft enough.) You can cut them in advance -- beeswax sheets are $1.50 per sheet near me. For sushi candles I can get 8 strips of the white out of one sheet, and about 16 pieces of colour out of one sheet. Or you could make votive type candles, just using one colour.
Bubble bath and body wash are simple if you use a pre-made base. (You can make bubble bath, body wash, and shampoo cold from scratch if you have the supplies.) Add some colour and fragrance, bottle it, and there you go. And pre-made lotion bases are about as easy. To make filling the bottles easier, buy some disposable plastic piping bags (again, if you are near a Daiso they are $2.00 for 18) and squirt the lotion into the bottles. It's so much easier than using funnels and way less messy!
Sugar scrubs are fun, but if you are going to buy a base, this might cost quite a bit. You can make your own non-emulsified scrub (loads of recipes on the 'net for this) and save some money, but it will take time for you to make it.
You could also make various things like cuticle oils (mix oils together, bottle), massage oils (again, mix and bottle), or hair oil.
Or you could make a perfume spray. I made one with my kids that was really basic -- 96% water, 2% fragrance, 2% polysorbate 20 (to emulsify the water with the oils). You can call it a soothing spray (lavender & eucalyptus) or a stimulating spray (something minty) or a cooling spray (peppermint) or a perfume spray (something perfumey). Put it into a spray bottle and you're done.
What about a clay mask? You mix some clays together, add water or a hydrosol or yogurt or something that suits the skin type and you've got a mask. For the make & take, you would just make the dry part and the student could add the liquid part at home. Cellophane bags to the rescue again!
Gels are fun too -- and they can be done cold. Mix water with the polymer gel, wait a few minutes, add the lye solution and you have gel. I have done glitter gel with the youth in my classes -- add as much glitter as you want, mix, package. They loved it! I've also done aloe vera gel with them for summer time.
I have found when I do spa crafts with my youth groups, they are big on the packaging and labelling. So make sure you offer labels and pens and stickers and other things like that for making one's own labels. There is something so satisfying about making up a name for your product and scent -- I think it really makes it your own!
As for colours and scents, it doesn't matter if you offer 3 or 30, people are going to complain that something they like isn't there. I would suggest going with a small colour sampler pack with the basics -- pink, purple, green, blue, and yellow -- and about 5 scents. I try to have a variety of scents -- one floral, one fruity, one foody, one basic, and one complicated (usually I go for spring dew, pineapple & cilantro, chocolate, lavender, and oatmeal milk & honey, but I do change it according to season and trend. Pink Sugar is very popular as well!)
Depending on the size of the package, you'll need quite a lot of fragrance. I do 2 ounce/60 ml bottles with my craft group of 20, and we use about 1 ml of fragrance oil per bottle. For bath salts, you would scent at 2%, so for 100 grams of bath salts (I would use this for 2 baths), you would want 3 to 5 ml of fragrance. Same with the bath bombs.
I hope I've offered some ideas. I love doing these things with my youth groups, and they seem to love it, too! (Sorry for the long post...) Let me know if you need some recipes and I can send them to you!