Pump the tires up to recommended maximums & learn to drive economically efficiently (Eco Driving Techniques) to minimise excessive climate change co2 plus carbon monoxide exhaust fumes for each & every journey to maximise fuel economy.
Enjoy your learning journey with the following tutorial videos.
Save fuel, save excessive wear on your vehicle, save the climate & best of all save loads of your money.
The vehicles original design engineers optimised all of the characteristics of Japanese K series cars for optimum overall efficiency, not just fuel efficiency.
Reducing excess baggage in the trunK/boot is advisable but removing any parts originally fitted to the vehicle & mods may compromise its safety & render it unpleasant to drive for little or no benefit.
Use a lighter battery remove the rear spare wheel replace with a repair kit and pump.
Not sure how much a car battery weighs but they are heavy to carry..
Rear seats if not needed.
Carry less fuel in the tank.
Find a weigh bridge to test be for and after total weight could be handy if you want to know how much lighter.
Other than taking the heavy rear seats out there's not much more I'd do to it at the risk of ruining the car. There's all sorts of carpets and sound deadening and plastic trims and spares you could take out if you really wanted to but it's just going to make it too difficult to live with day to day
The biggest thing that affects fuel economy is how you drive it. Accelerate slowly, avoid braking and just coast to a stop in gear, leave a big gap between yourself and the car infront, don't try to get to the speedlimit going up hill just find a gear that gives you enough torque to get you up the hill and hold it a one speed until you get to the top, keep the speed down on motorways - if you can complete your journey at 55 instead of 70 or 80 or 90 you'll save money (but waste time). Try to avoid journeys at rush hour and in stop and go traffic, always keep your tires at the right pressure, avoid overtaking. Just drive it like a granny basically and you can get it to 40-55mpg easily. Keeping up with traffic and driving like an average person will get you 32-36mpg, and ragging it around like a rally car will get you around 20mpg