Mk2.5 / Mk2.5S / Mk2.5S+
Printed part upgrades
Chassis, etc.
top-mount LCD brackets (makes printer more compact and easier to use)
triangulation beams (improves printer rigidity and quality for taller prints and maintains printer alignment when transporting)
clip-on storage trays (store tools and nozzles nearby)
printer feet (print in TPU/flex filament and combine with a concrete paver for reduced vibration/noise)
adjustable friction spool holder (some spools unwind too quickly, so this approach allows you to fine-tune the friction to make filament feeding more consistent)
SDcard shim (if your SD cards are not recognized, this may help keep the reader aligned with the PCB so it is more reliable)
Extruder & X-axis
Mk3S+ style extruder parts (easier maintenance compared to Mk2/Mk2.5S - good to have spares of all the parts surrounding the heat block)
upgraded part cooling fan duct for Mk3S+ (more even cooling on all sides - helpful for PETG printing where the fan rarely exceeds 50% speed; print in ABS or ASA and have a spare)
X motor cable strain relief (probably not needed, but routes the wires more cleanly)
extruder motor cooler (for long prints in an enclosures where ironing is used, the motors can overheat causing filament jams - a fan and shroud solve this issue at the expense of print speed due to the extra mass)
Y-axis
Prusa Mk3s Y-belt tensioner (two-piece design makes it easier to properly set belt tension)
Y motor mount (minimizes flex of the steel rods to maintain better printer alignment)
12" x 12"x 0.25" cork boards (insulates bottom of heatbed to reduce power consumption and reduce warmup times - can reduce print quality if running at high speeds due to extra mass)
Printer hardware upgrades
This PCB allows you to bypass the old Allegro A4984 stepper-drivers on the Rambo mainboard and re-direct the signals to discrete TMC2209 stepstick drivers to reduce noise and improve reliability/consistency.
You'll need
PCB from the Tindie vendor based in Sweden
soldering skills
A potential further upgrade is a DC-DC boost converter in between the PSU and this PCB to "upgrade" the motors from 12V to 24V so they run cooler and quieter. (Prusa Mk3 and Mk2.x use the same motors.)
Cost: ~$60-80 ($30 for PCB, $30 for stepstick modules, $20 for DC-DC converter)
Install: 2-3 hours
The Mk2.5S upgrade transforms your Mk2/Mk2S into a much more usable machine.
The key change is the removable build plate and magnetic heat-bed. The extruder has been totally overhauled to improve reliability as well. You'll need to print a bunch of new parts in PETG. Another benefit of this hardware upgrade is that it allows using the more modern Mk2.5S firmware that has hundreds of improvements comapred to the Mk2S which stopped getting updates in 2019.
Cost: ~$100 plus printed parts
Install: 2-3 hours
If you have a Mk2.5, consider upgrading your filament sensor to the more-reliable "S" style. The only drawback with this sensor is that it only detects filament runout, not that the filament is actually advancing or retracting. You'll need to print out S-style or S+ style extruder parts: https://www.prusaprinters.org/prints/61346-i3-mk3s-to-mk3s-upgrade-printable-parts
Cost: ~$12-15 in parts plus $3 in filament
install: 30 minutes (plus print time)
The texture is similar to the Prusa sheets but results in a more porous looking finish on PLA and PETG. On Reddit and Amazon, customers claim that it is easier to print with these sheets compared to the original Prusa parts. Regardless of which textured sheets you choose, your parts will look far more professional with a textured sheet than regular smooth sheets, but some added care is needed:
Use 90%+ isopropyl alcohol (IPA) to clean after each print
After 10-20 prints, consider rinsing with mild dish detergent and warm water, then dry and wipe down both sides with IPA again.
When releasing larger PETG prints, allow the part to fully cool to room temperature, then consider heating for a few minutes prior to releasing the print.
For trickier PLA and ASA prints that tend to warp or dislodge, it is OK to use PVA glue stick to fill in the gaps on the texture. This will help the print stick and release when finished. Note that the texture of the part will look a little weird and there will be white residue on both the part and the sheet which you'll need to clean off with warm water.
Cost: ~$30-40
Install: 2 seconds
An enclosure helps to maintain a consistent air temperature around the printer resulting in more reliable prints for PETG and ABS. It also reduces noise and smells. You will need to cut a small hole to pass the power cable through.
Cost: ~$30-40
Install: 5 minutes
Ever get blobs of filament sticking to the nozzle? Or have a difficult time keeping the extruder at a constant temperature when the part cooling fan is on?
Put a sock on it!
These little beauties just clip onto the heat block and keep everything clean and at the proper temperature. They don't last forever and will eventually crack or deform from so many heat-cycles, so best to keep a spare.
Cost: ~$3 each
Install: 1-5 minutes depending on which part cooling shroud you have installed
The default 0.4mm nozzle is a "jack-of-all-trades" it does everything OK but isn't the best at any particular job. Don't be afraid of swapping the nozzle on your printer. Use the right nozzle for the right job. Need super detail for a figurine and can wait all day? Try the 0.25mm nozzle. Need a super strong part that prints fast? Try the 0.6mm or 0.8mm nozzle.
Cost: ~$5 per nozzle
Install: 5-10 minutes depending on which part cooling shroud you have installed
Spare parts
Some printer components wear out so it's best to have a few spares:
LM8UU bearings (Misumi brand are best but more expensive and often out of stock)
extruder motor (3DO or Moons brands are best, but generic is OK)
2mm PTFE tubing (may be required when re-building extruder)
all extruder plastic pieces - these can be damaged if your print fails and a massive blob of melted filament envelopes them