i just did my first con like half a year ago!
we did mostly small cheap stuff since we didnt know what the return would be; posters,stickers, on-the-spot commissions.
my only recommendations are
a. dont go alone! it's stressful as hell. we went to a super small con and barely got by with 3 people on a 1m table
b. consider what con you're going to. we went to an anime con and predictably, didnt sell any original stuff. similarly i wouldnt try selling anime merch in a fantasy con, etc.
c. keep track of your income! we tried out best with ours but still ended up with more income than we accounted for somehow.. either way this helps a lot if you're planning to do future cons and want to know what to put the $ into in prep
d. consider the size of the con. i once heard for every 1k people in con, 100 will go by your table, and 10 would buy? something like that? i cant remember if thats accurate at all, but still best to keep in mind as much as you can. don't over-prepare for a small con, or under-prepare for a big one. before we did out first tabling we reached out to artists we know who tabled there before and milked them for recommended sale amounts
e. price according to what everyone is. roughly, of course. itll help you not overcharge (so there arent enough buyers) or undercharge (for your own sake). a dear friend of ours was tabling at the same con doing original works and slightly higher priced and despite being way more prepared, made about 1/4th what we did. its all about matching yourself!
for us stickers sold the most, then stickers, then small posters, then big posters. basically small to big, make of that what you will haha
* on-the-spot commissions were slot limited but still made a good amount of cash, about the same as stickers overall though obviously less in quantity