what can i do on a datsun 120y WAGON

Well Steve, i dont know how long you have had it, so i will assume that it is a new aquisition.
1. Give it a thorough clean, inside & out. This will allow you to see what it is you really have bought. If you are still happy with it, then.
2. Give it a complete service & tune up. This way, you bring your variables under control. After this, you will KNOW about the mechanical condition of the car.
3. Cary out any minor repairs that you found necessary during the service. Brakes a bit worn? Now's the time to fit new linings. A trip to the wreckers will often turn up many of the parts you need, like lenses, knobs, trims etc.

Now you will have a clean, sweet running stocker. Check out what wheels are available. A set of 13" mags will dress it up very nicely & you will have a good street hack that awaits your further attention.

There are lots of options, like, A14 / A15 engine upgrade, paint, interior, further engine mods & lots more. Just make sure that the body shell is sound, & worth the effort before spending any real money in other areas. If she's straight & sound, the sky's the limit. Keep tuned to this site for lots of ideas.
Chris

it was my dads work car so i can drive

i think the bigger question is, what CAN'T you do...!
a RWD chassis that weighs a shade over 750kgs is open to many possibilities.

1) big brakes. all on stanza struts, but i have seen VL discs used on 120Y struts.
2) diffs. anything. honestly......anything.
3) engine. take your pick. rotary, SR20, CA18, VG20, VG30, rover V8, 4VEU 3.4L toyota V8, 3.8L buick V6....
4) interior. limited to the size of your wallet.
5) suspension. coil overs, 4 link rear ends, lowering blocks, stanza struts, adj. strut tops etc etc.

how fast do you want to spend?

Fit a set of 13 inch mags, a set of bolt on flares and lower it 2". Then maybe some flows and a 32/36 webber. And perhaps tint out some of the rear side glass.
All that shouldnt set you badk too much, and wont effect the relability too much .