This is awesome for someone still in college! Congratulations on being so smart so young. You are going to become very, very wealthy if you can keep this up over the long term.
A few questions/thoughts about how you might maximize things further:
1) Can you give some more detail about the 529 plan and your expected college expenses from here on out? You say you have roughly 10k left in the 529 -- what do you have left to pay for school? Unless you are planning to go to grad school, you might as well cover all the school-related costs you can from that, unless you have a plan to turn the balance over to your spouse or kids in the future (not a bad alternative plan, btw). Anyway, wasn't clear from how you described it how exactly the numers would be playing out there, so more information might help us help you plan the use of that asset.
I don't really know any details about the 529 plan other than the approximate value and that my grandmother started it through the Connecticut Higher Education Trust. I have yet to tap into it for any college expenses. I am planning for six more semesters of school (not sure of specifics quite yet, but that's what I am planning for), starting next fall. I would like to use the 529 money last if at all possible, because a) it will have more time to accumulate interest, and b) it'll make my grandma happy. My estimate of the total cost of those six semesters is 25 grand, so I would like to have about 15 saved up for school.
2) Since you are now working full time, is there any possibility that your employer could cover part of your remaining school expenses? I know probably unlikely, but it doesn't hurt to ask.
Extremely unlikely, given my current job and my future degree are in rather different fields (chemistry vs. electrical engineering).
3) Living expenses. Looks like you are knocking it out of the park with a sensible housing purchase and excellent cost reduction strategies. Is the sharing arrangement likely to continue once you start your "real life?" As long as you all get along well, I would stick with that for as long as you can -- you ALL will become wealthy if you can keep living costs this low in early adulthood
We plan on sticking this out as much as we can until we all have our lives in order. I would also like to save up a tidy sum before selling this house.
On the eating out thing, could you propose an arrangement with your partner where you cut the number of times you eat out a month in half, and for your half you promise to make her an awesome special meal? It will probably still cost 1/2 of what it would cost you to eat out, and you could then up savings.
She's the awesome cook, not me unfortunately. Still something I can work on though.
Gas -- is your water heater insulated? If not, an insulating blanket might help drop the cost. If everybody is gone during the day then a timer on the thermostat might also save money (have it come on early morning and then early afternoon, shut off after everyone typically leaves the house and goes to bed).
Had not ever thought about insulating our water heater. That is something I will investigate today. What sort of insulating blanket is best for this application?
We do try to turn the thermostat down when the house is empty, although sometimes this is forgotten as it can be hard to keep track of. Also, we don't all work similar hours, so there isn't always a lot of time when the house is empty anyway. I don't think our thermostat is compatible with any such timer. We have an old school Honeywell dial thermometer.
Electricity -- see if you can install ceiling fans before next summer. they are awesome. We spent a horribly hot summer in a NYC apartment with nothing but ceiling fans and did fine. MMM's article about conditioning your body to adapt to hotter temperatures is right on the money, in my book.
We have a ceiling fan in one room, and it works great. The other rooms of the house unfortunately have rather low ceilings and I think ceiling fans would be asking for trouble. As the tallest member of our household, I already tend to hit my head on our protruding overhead lights, to the great amusement of others. My girlfriend and her sister are from Phoenix, and conventional wisdom would say that they are used to the heat. However, in reality, they are merely used to air conditioning all the time. First thing either of them do when we get in a car is turn on the AC. Adaptation conditioning would be a hard sell to them.
Gas: Once you aren't doing school + full time job, will it be feasible to use public transit or bike to work? Understand moving closer is unlikely with your sweet housing deal, but maybe you can reduce the cost of gas, even a couple of days a week. Carpooling would also help with this.
This is something I've thought of even with my current set-up. I need my car for work occasionally, but I also have a train that has a stop about a quarter mile from work. The stop I'd have to get on is four miles from home. I've thought of leaving my car at work during the week, just driving it in Monday and home Friday, and biking to and the train station during the week. A train ticket round trip works out to be about double what I pay for gas in my car, so it's kind of a toss up whether it would be worth it to not have my car at home weeknights.
Gym membership: What do you actually do at the gym? Are there ways you could get similar exercise without the recurring cost (e.g. running outside, biking outside, investing in a set of weights (can be purchased on craigslist or ebay) for working out at home)?
This is more a motivation thing than anything else. If I pay for it, I will go because I want to get my money's worth. My girlfriend's sister's boyfriend is helping me get into certain exercises there. I'm still pretty new to it, only had the membership for a few months. I'm nowhere near overweight, but hardly in shape. We don't really have any room in our house for any exercise equipment, even something as simple as some weights. Also, it is nice to get a decent shower at the gym, as our well water is very hard and harsh on hair and such (the female members of our household have gym memberships for solely this reason. Working out is just a bonus for them).
Overall it looks like you are doing great. Not sure what kind of groceries you all buy, but the cost seems very reasonable for 4 people. However, if you aren't doing so already you might be able to bring that down by judicious visits to costco, ethnic markets, co-ops, etc.
We've been exploring this lately. Also, a coworker of mine has gotten us into some couponing which is starting to save us a decent amount. We try to balance cheapness, healthiness, deliciousness, and easiness, not always in that order.