How do I decorate this small front porch for Halloween/Christmas?

That could work. You can buy (or find) corn stalks for a nice fall theme.

You can get a couple of large square pavers from a home warehouse store (Home Depot) and set them on either side of your front concrete. Then get a couple of large pots or urns and pop in what you would like to see in them. You could plant some live plants, or can fill them with some fall mums (faux or real) or just fill them with faux fall leaves. You could also stack pumpkins in them, again, real, or artificial and tuck fall leaves in around the bottom of the pumpkins and in between them. If you want to add a wreath, you can use a Command hook to attach it to your glass doors. With the pair of doors, I would probably go with a smaller fall swag on each door. If you don't want to put something on the door, you could put it on your tree there, attaching it by tying a piece of twine around the tree. Even a Dollar Tree fall sign would look nice on your tree! Good luck and enjoy!

You could do a glowing pumpkin totem pole - https://www.pinterest.com/pin/318348267405962330/

Throw some skeletons into your trees for creepy decor - https://www.pinterest.com/pin/318348267405962293/

And my personal favorite for Christmas. My mom did something similar but with different size plastic decorative balls. https://www.pinterest.com/pin/318348267406831925/

4-6 cement pavers or a couple of bags of low cost gravel will work better than the pallet with the pallet lots of leaves and bugs will get trapped inbetween boards

The pallet idea is good, if you can find one that fits the space. I had the same situation at an entry, plus I needed something people could step on, so I got some large square pavers from Home Depot or Lowes, and added them right next to the existing stone porch. You can often get pavers very cheap, as little as a dollar apiece. You could get either plain gray which will blend in, or some fancier design to make a noticeable focal point where you can place your decorations. Be sure to get some sand to use as a base for the pavers, so you can level them off and perhaps make them the same height as the porch, if you like.

Then you can choose whatever sort of planter or holiday-themed item you like. It appears you've got an electric outlet there, so you can do anything! Or place a small spotlight or solar light to show it off at night.

I would only do some planters with fall flowers or pumpkin topiaries ...less is more

As the tree is the largest feature, I say embrace it. I think that rope lights wrapped around the trunk would look stunning, in any season.

Also recommended for tree trunks are tree net lights. Depending on where you live, these are sometimes called ribbon lights.