If you are going to ride it as long as you can stand it, beyond lubrication I would consider:
-new brake pads if the old ones are hard
-truing the wheels if needed
-changing saddles if needed for comfort
-shifting adjustments for reliable shifts
-Possibly a rear rack and trunk bag
-Possibly different stoker bars if your stoker is not comfortable with position due to current bars.
-A good bright blinkie
Not a huge investment and you could probably move the saddles, rack, stoker bars, and blinkie to a new bike.
Keep an open mind and focus on the fun, then when you upgrade, the fun will just get funner.