Stubby bar ends on an S-type.
Firm suspension block with a jubilee clip round it too.
SPDs.
Bottle bosses (use Elite strap-on ones on the back of the "stem").
On older ones with the swaged chainring, fit proper cranks with a bolted chainring. A folding pedal will not go up and over the end of most cranks so you may need to stick to Brompton's own left crank.
Closer-ratio 3 speed gears. Screw an AM mechanism into the SRF3 shell.
Better grips.
Better saddle.
Rebuild the wheels, which are usually undertensioned and not very round.
Buy the seat height limiter unless you are lucky enough to get the correct seat height just by pulling the post right up.
Eazy Wheels or skate wheels. The stock wheels will flay your calves if you have well-developed calf muscles.
Avoid:
Taiwanese titanium seatposts. Light but alarmingly flexy.