The reason your code is not working is because you need to do the math before you convert to a string. Essentially, this is what you are telling the compiler:
"Monday" + 2
That doesn't make any sense in math terms, so it throws an error. the sysdate is actually already a number, so you can add another number to it. Then, you use your to_char to convert that single value into a formatted string like this:
create or replace function get_return_date(
p_days in number)
return varchar2
is
begin
return to_char(sysdate + p_days,'YYYY-MM-DD:HH24:MI');
end;
/