Hijack the ISS to Mars
Equip ISS with low-G thrusters, do orbital fueling to reach Mars much sooner.
We really ought to have gone to Mars by now - we've lived here for several billion years, and so far we haven't even visited the neighbours - how rude.
One of the problems with going to Mars, of course, is that it's quite a long way to go - more than a hundred times as far as the Moon. Building a ship that's capable of hosting a crew safely for many, many months is tricky.
At the same time, though, we have the ISS just floating around in Earth orbit. We know that people can survive on the ISS, fairly comfortably, for very long periods of time. The record for space endurance was set on Mir (which was smaller than the ISS), and was about 14 months.
The ISS is also pretty good - way better than regular spaceships - in terms of redundant systems. It's big enough and complex enough that the number of system failures is fairly constant, and most failures are not fatal. There have been fires, leaks and numerous breakdowns on the ISS over the years, but none (yet) has proven disastrous. In contrast, many such incidents on a small, regular spaceship would have been potentially fatal.
So, here's what we do. First, we send up a whole bunch of boosters to the ISS and bolt them on. We also bolt on a Mars lander/ascender or two, for later. Oh, and a few extra containers of food, water and oxygen. (The ISS is quite good at recycling water and air, so it won't need vast reserves of these.) All this can be done over the course of months or even years, riding out fluctuations in budgets.
Next, we fire up the boosters and point the thing at Mars. The boosters will have to be very low-thrust, long-burning rockets, because the ISS isn't designed to withstand large accelerations - but that's OK. If the thrust is applied over a few days or even weeks, it'll be fine.
Once we get to Mars, we pop the ISS into Mars orbit (again using a very gentle acceleration to do so), and then hang around for a few weeks looking at the scenery and checking out systems. When we're ready, we hop into the lander and nip down to the surface to do a bit of exploring. The crew can be large enough that a team can stay in the ISS while the other lot are out shopping.
The return mission is much the same as the outbound mission, but in reverse.
The advantage of sending the ISS is that it's a large, well-resourced and well-tested ship with proven ability to sustain life for months at a time. And, when it gets back, it can go back into Earth orbit and carry on as before.