Seal the scuttle grommets, check your rear drainage tubes are clear and check that your thermostat’s ok; gauge should be a good half-way up the dial, better still get an obd2 dongle. Lots of info on the forum about all these things.
Fit a wiper switch from an Rx8 - this features the variable interval intermittent wipe. For some strange reason Mazda fitted the MX-5 with a single (fixed) interval intermittent wiper switch, whilst the Rx8 got the rheostat…but they’re “plug & play”, so an easy mod.
Check the other fluids and consider brake / clutch fluid change.
Thermostat check as mentioned earlier.
Check your tyres. If they’re older than 7 years, or not matched all round, get a fresh set. Something like Yokohama is a good mid ranger.
Upgrade your front brake pads and check inside wall of discs for wear/rust and change these at same time. Brembo is a good brand but there are many posts on here with equally or better recommendations.
You’ll find the ride improvements are huge just by doing the tyres. If you want to take it to the next level you could install coilovers or lowering springs but that’s a bit more technical and not for the faint hearted!
The beauty of the MX-5 is you can do a lot of work yourself with minimum experience. There’s plenty more you can do to make it a bit more sporty such as louder exhaust, wheel hub spacers; all doable yourself.
As others have mentioned the RX-8 wiper stalk’s a great (and easy) mod
Oil, filter and spark plug change if you don’t know when they were last done
Depending on the mileage auxiliary belt change
Swap the coolant expansion tank for an aluminium one (or just a brand new plastic one)
Get underneath it and have a good check all around for rust especially on the subframes and rear sills .
Finally get an MX-5 OC sticker on it, adds an easy 50bhp and makes the car look 100x better.
Once all of the essentials are done sit down have a good think about what you want from the car, do you want something that’ll turn heads? Do you want to go down the forced induction route? Do you want a car that’s capable on track? A mix of these? Once you’ve decided what you want it to do start looking into your options for what you’ve decided and go from there.
I’d also pick up a copy of Keith Tanner from Flyin’ Miata’s new book How to Build a High-Performance Mazda Miata it’s brilliant and explains all the most common modifications and what they do and in some instances don’t do.