Modifying existing greenhouse into something useful

I inherited two dinky little greenhouses from my grandfather.  They are the standard, 8 foot by 10 foot, peak roof, glass contraptions with one (too short) door on the end and an itty-bitty window on top.  Although I can manage to grow hot peppers and cotton in them, they aren't very good at it.  There's not enough airflow to keep the plants happy.  If the sun is out, they get too hot.  At night (we have cool nights), it gets too cold.  The plants don't like these temperature fluctuations.  

Is there some way to take these greenhouses and transform them into something useful?  I was thinking of putting some sort of heat sink in them like paving stone pathway.  But maybe something more drastic would work better.  

A heatsink would help, but most important is probably to increase the airflow. Can you make some changes so that you can remove the ends completely for the summer? It sounds like the door and window are good for opening in the daytime in the shoulder season of spring and fall, but when it really gets hot it would help to open the two ends completely. Failing that, a powerful exhaust fan at one end would help some but maybe not enough.

You described the challenge with greenhouses perfectly -- way too hot, way to cold.... A big reason why the dinky greenhouse versions don't work very well as year-round growing structures.
To combat overheating, I would add fans (though you have to get electricity to it). For growing in the summer, perhaps you could remove one of the panels of glass for more airflow?
To combat freezing at night, I would try to seal it up as much as possible by caulking the seams. I am guessing they leak like a sieve which won't do much for retaining warmer temperatures at night.
Then, yes, thermal mass is a great idea. I would consider water barrels if you can fit them (perhaps under growing benches?). Just wrote a blog on water barrels in greenhouses if you want to see some pics of others (though obviously you don't have to go as far as a water wall; simple jugs can help a bit). http://www.ceresgs.com/tips-on-using-water-barrels-in-a-solar-greenhouse/

Several issues here. Like most GH's you're getting too much heat in the summer and not enough in the winter. Venting excess heat in the summer is the typical solution and you will likely need some regardless of what heat storing solution you put in.  Putting shade cloth over the GH to cut down on sunlight and heat during the summer is another common solution, as is taking out panels and/or leaving doors open in the summer. Venting can be a DC fan which runs on a small PV panel if you don't have access to AC power too, or vents which open with heat sensitive actuators. You can also look into storing the excess for nights and cold days. Barrels of water is one way, but it tends to use up precious GH space. You can look into SHCS, which store heat in the soil under the GH. The SCHS might work well if you are planning on moving the GH to a new location, since it requires burying tubes in the ground under the GH. Be aware that in most cold climates, it is not possible to store enough excess summer heat to get thru the entire winter.

For the colder months, you probably will face the opposite issue, how to keep heat around longer. You may want to look at an inner liner in the GH to help with losses thru the glass. And insulating the foundation and/or soil under the GH will also help. Auxiliary heat is an option too, but can get quite expensive and/or time consuming if you are doing it manually.

I am not familiar with your climate, so can't really speak to what might be possible or not for a GH there, but you should be able to get the figures for solar radiation in the various months, degree days of heating, etc and calculate how much heat loss you will experience at various outside temps, and how much heat you'd have to add/store to keep the GH at some temp you desire for your plants (if you are planning on using it over the winter). Elliot Coleman's book on winter vegie growing talks a lot about how to manage a GH for winter production with little or no energy inputs.

It is a doable project, making small GH's usable. Not necessarily all 12 months of the year everywhere and not necessarily without some types of additional heat or storage in some places. But even with these limitations, a GH can add significantly to your ability to grow plants.

These are great ideas.

One of the things I was wondering is if a larger greenhouse would be better than two smaller ones?

I'm already thinking of changing location so that they get more sun in the winter.  As they are now, they are shaded from 10am to 2pm in the winter (our winter daylight is roughly 9am to 4pm).  It has direct sun for most of the day in the summer, except about two hours after sunrise.  Usually I grow something like squash at the base of the greenhouse.  These grow large enough to provide partial shade but I can cut the leaves back if they get too big.

Would it be possible to take these two greenhouses and create a single greenhouse?  I don't have a south facing wall I can put them on, but maybe something could be done?  

Can you disassemble them and use the parts to make one large south wall on a framed and insulated structure.  What I have discovered is that at this latitude having a glazed roof is useless and a glazed north wall is a negative. And I am at least one degree south of you.  The west wall works best as a trombe wall heated by the evening sun in the summer to ward off the morning chill. I will repost the picture of the one I put up quickly the first fall to protect the plants in barrels.  I used discarded carpet and padding for the north wall which I can roll up for ventilation and one section has shiny aluminum roofing as a reflector on the inside of the north wall.