There are lots of great finishes that you could use, the choice you end up making will depend on how hard you want it to be, how shiny you want it to be and how much abrasion resistance you’re after.
I’ll preface this by saying that leather gets scratched, that’s just a fact. You won’t totally be able to protect it from little harms that may come along the way. If it’s an item that will be seeing a lot of heavy use, you may consider using a leather that is more naturally abrasion resistant first and then sealing it well.
Now, there is a post all about finishes coming up soon, so we’ll try to keep this answer as simple and specific as possible.
An acrylic finish is what we suggest. It will keep your work flexible and give you some abrasion resistance, as well as seal in any color you may have added. You can build the acrylic up for a thicker finish, but you’ll want to do it in thin layers to avoid cracking and peeling.
If you’re looking for more hardness, you might try a polyurethane based finisher. It is also relatively thick and will stiffen/harden your project considerably – the degree depends on the ratio/product you’re using.
Oil based sprays like Master’s Quick Shine will give you a high gloss finish and an excellent seal, but won’t do much in the way of protecting your work from scratches. Lacquer will give you similar shine but can also be susceptible to scratching.