VASIMR looks like the best bet so far, since it can go to high thrust in a gravity well to maximize the Oberth effect, then switch to low-thrust, high specific impulse mode for the cruise. But to power it you'll probably need a liquid fluoride reactor, which has a good power to weight ratio.
If you intend more than flags and footprints, you'll also need to spin the ship for artificial gravity, because although humans can somewhat adapt to zero G, very few other things can. Transporting crops and livestock (chickens and fish if nothing else) is extremely problematic without gravity to keep feed and poop from floating around. Plus, almost everything involved in daily life (cooking, washing, etc) is much easier with gravity.