off the top of my head i can't think of a long list.
Seratonin syndrome:
DXM, anything MDMA/mephedrone related that would release serotonin (won't get into which specific ones unless you want me to)
Meth/coke are debatable --> more rarely seen
Diminished effect:
LSD, shrooms, some block nicotine receptor, etc.
Unwanted effects:
Alcohol (you may get drunk with fewer beers --> nausea, vomitting)
But you have to remember that the common side effects for all SSRI's are nausea/vomiting and headache
As far as increasing the amount of the drug in the body that could lead to side effects, it really depends on the SSRI. Escitalopram is one of the safest ones because it doesn't strongly inhibit the CYP enzymes like other SSRI's do (ex. Fluvoxamine, paroxetine,...)