1. The Laws of Physics. Time machines aren't actually possible. Dull, dull, dull.
2. Crossing the State Line. The The creation of a functional time machine causes a state change in the Universe. The first part of the Universe is temporally fixed, but the second part is temporally mutable. You cannot cross the temporal schism boundary without causing a rift in the space-time continuum, so you can only time travel within the period where time travel is possible.
3. The Teenage Asshole Paradox. It is inevitable that at some point after the invention of the time machine, some kids will think it's funny to nick their dad's kit, take it for a spin, and kill the inventor, which has the same effect as killing their grand-dad. So every time it gets invented, it gets uninvented.
And I want mine to look like this.