Too nice of a boss...for a reason?

Is there such a thing as having a too nice of boss and knowing that they will expect free/extra services in place for their kindness? If so, what is the best way to kindly decline their "over the top" offerings (like a car, trip to Vegas, gifts), and also what are ways to address the situation directly. I have my own web design business (2 person team) and recently accepted a "temporary position" for one of my clients who was in desperate need to fill an unexpected void in their small 5 person team. I have only worked there for a month, now putting in 9-10 hours a day 5 days a week, and they have been showering me with gifts, trip to vegas, insist that I drive their car, clothes, and other expensive gifts. In this time period they have implied that they wanted me to do the web work in-house, but it is less than 1/2 of my normal company hourly rate... but I feel bad telling invoicing them because of all of these things that they are doing for me and giving me. It is so absurd that even when I make it EXTREMELY clear that I do not need the car,clothes,gifts and that I would not be able at this time to take the trip, they insist on "making it work for me" no matter what excuse I give them. Sorry post is so long, but I really need advice.

Tell them that you're sorry but since you run a business that other people depend on for profits, you are only able to accept money in return for services rendered. Tell them that although it's nice that they want to offer you gifts to show their appreciation, you would like to keep things on a professional level.
It might help if you give them a few examples of "bonuses" you could accept if they really feel they need to do something to show how much they like you. Things like references/referrals for your business or perhaps getting you some kind of assistant to take the load off your shoulders...this might also help because they could pay the assistant less money to do things to help you get done faster and you can bill them for less time and have more time for your other clients.
It sort of sounds like they're trying to make you part of the family. I don't know if that's good or bad. Maybe you could just tell them what you told us...that offering these gifts to you makes you feel like you are not an employee and it makes you feel funny about charging them the fees you have EARNED...but that you as a businessperson can not afford to work for free (or less than you are charging).
Another option would be having them pay you more of a salary than an hourly rate, that way you won't have to give them an itemized bill each pay period. Make sure you make it a good salary! But that would get you out of the uncomfortable position they've put you in of having to bill them each period and then feeling like you're asking too much after the "gifts". Then if you feel like it, go ahead and accept a few gifts if you want to! They're offering, so you must be worth it! If it gets to be too much for them, they'll stop offering.

He is trying to trick you into killing yourself!

the best way to decline? report them to the dept of labor for sexual harrassment and also to the district attorney. one reason they may be giving you all that stuff is because they are hand me down gifts. if they can't pay you, then they can't afford to buy those gifts. they had to get them somehow and i am guessing someone gave them to them. tell them you need time off for an unexpected family matter (your own) but ask them to "hold" your position. after being away 48 hours call and say you will have to ask for a long term leave of absence. in short, get them to fire you!

Tell them that although you appreciate all the "extras" your real need and desire is to make your company work and to reap the income from that business.
You'd prefer to have the cash instead of these extras.

Well, It seems they are desperate to hire you. I would attempt to negotiate better terms if your interested. If your not interested, make it abundantly clear. They are trying to win you over.
You could also make it known that the gifts are inappropriate and firmly decline. But you have to ask yourself, if they are willing to do all these things, would they consider matching or beating your current wage?

Simple. Take the schwag and ask for more money. Be clear that you'll accept the car and all that as gifts, but it doesn't factor significantly into pricing because it can't cover payroll for your employees.