If you don't mind waiting millions of years to accelerate, You can do CMB photon sailing: thermodynamics of a dual-face surface in spacethermodynamics of a dual-face surface in space
There is also spacetime swimming: Can a deformable object "swim" in curved space-time?Can a deformable object "swim" in curved space-time? which is (from what we know) also really really slow, but satisfies your demands