The most worthwhile improvements include
- A lightweight stool or chair to enable a comfortable viewing position at all altitudes.
- Contrast enhancements - Flocking the upper inside of the OTA opposite the focuser. Flocking the lower tube portion thats visible from the focuser. Front ota extension.
- Movements - milk bottle washers around centre bolt, pimply laminate on underside of rocker box waxed with turtle wax paste car polish.
- Right angle finderscope to save your neck, coupled with a Rigel or Telrad finder.
- Balancing - Large speaker magnets covered with cloth or felt to prevent scratching
- A mirror cooling fan
- The biggie! - An argonavis
Threads already exist on all of the above. A bit of time using the search function will throw up a lot of useful info on these.