insert into Activities (person_id,act_date,act_type,notes,next_contact) values (
I add values for each column, but if there's no scheduled next_contact date I left that off. To me, this looks like your second example (with two columns of values and no date) and I don't see the differences.
Assuming you're meaning this example:
INSERT INTO foo (a,b) VALUES (1,2);
The difference is I didn't specify field c in the list of columns, so it gets inserted with its default value. That would be the same as:
insert into Activities (person_id,act_date,act_type,notes) values...
Now that will work if you're doing a separate INSERT for each row. If you're doing multiple VALUES in one select, and some have a next contact date and some don't, then I think you're going to need to explicitly spell out your NULLs.