Now obviously this isn't going to be much use for takeoffs and landings, and I can either go with auxilliary engines that have higher power output but can't be run for a sustained time, or with an "emergency / takeoff / landing" setting on the main engines, like using the afterburners on a jet. A lot more power, but it burns through fuel extremely fast, say 200 times as much fuel for 100 times as much thrust. E.g. to get 2g for 10 minutes uses as much fuel as .02g for 2000 minutes, about 33 hours. And 33 hours at 0.02g will take you a LOT further than 10 minutes at 2g.
Does this sound reasonable for game purposes?
Later: To clarify this, my best guess for travel times is a few weeks for Earth-Mars and Earth-Venus, with Saturn to Neptune and beyond just on the edge of the possible. In The Red Peri they imply that Saturn-Pluto (about a billion miles) can be done in three months or possibly less, but they don't give exact times.
Assuming that it'll take three months for the 1 billion mile trip under normal circumstances, that's about 0.01g cruising acceleration /deceleration. Plugging in a range of values gives 1 billion mile trip times of:
0.01g = 93.7 days
0.015g = 76.5
0.02g = 66.3
0.025g = 59.3
0.03g = 54.1
And Earth-Mars times at conjunction of:
0.01g = 20.2 days
0.015g = 16.5
0.02g = 14.3
0.025g = 12.8
0.03g = 11.7
I think I'm going to go with 0.01g for the earlier ships in e.g. A Martian Odyssey, 0.02g to 0.03g for later ships.