See Quantum Field Theory for the Gifted Amateur by Lancaster and Blundell.

Exercise 1.1 demo Derive Snell's law
Exercise 1.2
Exercise 1.3
Exercise 1.4
Exercise 1.5
Exercise 1.6

Exercise 2.1
Exercise 2.2 Perturbation example
Exercise 2.4

Exercise 3.1
Exercise 3.2

Exercise 5.1 Total time derivative, Euler-Lagrange equation
Exercise 5.2
Exercise 5.3
Exercise 5.4

Exercise 7.1 Equation of motion from Lagrangian
Exercise 7.2

Exercise 8.1
Exercise 8.2

Exercise 9.1

Exercise 10.1
Exercise 10.3

Exercise 11.1

Exercise 12.1

Exercise 13.1 demo Levi-Civita symbol; scalars commute with operators
Exercise 13.4 demo

Exercise 14.2

Exercise 16.1 demo Propagator for particle in a box

Exercise 36.1 demo

Exercise 37.1
Exercise 37.2 demo
Exercise 37.3 Derive Dirac equation from boost