The first place to start is with safety. Hand bars, grab rails, banisters...If you have an extra room (say a kid grows up and moves out) you can combine two rooms to make one large one. Widen doorways is a big project and requires larger doors to replace the ones you took out, but it makes it so a wheel chair can pass in and out of rooms. Put the hook door handles on instead of the round knobs so the level is pulled to open a door. Use slider switches on the lights and lower them a bit so they can be reached from a WC without you having to lean way up and out of the chair to turn them on or off, or get remote controled ceiling fans with lights and use that. You can have your tub removed and put in a roll in shower (bath fitters) will be much easier to slide from your chair to the stool in the shower, unless you want to install a lift to pick you up from your chair and hover you over the tub and then drop you in. They dont take up much room.

Remove slick floor stuff and put down fall friendly floors. Carpet may be harder to take care of in some areas, but if you fall it cushions the blow. Hard woods are easier in the main areas to roll your chair into and out of. Replace appliances that have knobs on top, with knobs on the bottom or front so you can reach um.

Tons of ideas on google. Some really cheap, some not. Good luck.