10g tank
small titanium heater with thermostat
HOB filter, with sponges you fit over the intake for dealing with weak or very small fish
basic low wattage light (for fish), more light as appropriate for corals
a few plastic plants and some pieces of PVC tubes, elbows, etc. (hiding places for fish, PVC pieces can be used to rest corals upright)
Plus have on hand:
Amquel+ Plus
basic water test kit
SeaChem Ammonia Alert badge
good quality activated carbon
air pump
airline
backwash valve
air stones
Furan-2 (broad spectrum antibiotic) OR Maracyn (gram + bacteria) and Maracyn 2 (gram - bacteria)
Praziquantel (internal parasites)
iodine (for corals only)
BBS eggs and mesh strainer