On the hand towels (as with all towels) I use a sheet of water soluble on the top of the towel when hooping, this keeps the "pile" flat when it is being embroidered. You can then pull it off after, if a bit is left, don'r worry as it will come out in the first wash... or you can dampen the area that still has the stabiliser with a bit of warm water to get rid of it, that is, if it is a gift.
On the t-shirting, I have used straight tear-a-away, interfacing (can be trimmed at back) I think the spray also works ok... just don't pull the t-shirt fabric too much as it will look like a puckered mess. Someone else may have some better ideas for the t-shirt, but with the towels, I have been doing embroidery on them for 25yrs & if I didn't even have washable stabiliser I would use plastic.... you just have to be careful you don't pull threads at the same time as the plastic... lol, you end up with thread loops everywhere otherwise.
Have fun,
Reggie

Here are some Cutie tips to help you with all this. If you look to the left of your post you will see lots of words in blue, go toward the end and click on tips to see if there is something there on t-shirts also. Terre

Here are 2 picture tutorials about how to stabilize T-Shirts or knit fabrics and how to stabilizer towels.
There are pictures at the left side that you can click on to see a closer look.

This is the Towel Tutorial

http://www.rnkdistributing.com/towels.php

This is the Tutorial for Knit Items such as T-Shirts.
http://www.rnkdistributing.com/knit.php

Both the Tutorials use Floriani Stabilizers but you can use other Brands of Stabilizers that have similar products as used in the tutorial.

EastWitch2

Embroidery Library also has some tutorials.
Here is the link to them and just scroll down the page until you find the ones for towels and T-shirts.

http://www.emblibrary.com/el/elprojects/holder.aspx?page=techniques

EastWitch2

This is the way I do it:
when I embroider a T shirt I always use sticky tear away in the hoop and put the t shirt on the sticky site.So no hoping of the shirt.
A towel.I hope the towel and use WSS on top of the towel so the fabric stays flat when i embroider.
I've never used spray adhesive myself.The woman of brother(I have a brother) who gave a demo wasn't very fond of adhesive spray.Good luck with your projects.