My car worked great as the transition from the house to the garden. Use plastic totes for easy unloading when you need to haul around people instead. Also keeps my car seats clean. I spread a towel or blanket just in case because on some days I was able to put the plants directly out on sheets of cardboard for some direct sun as I increased the amount of daylight. I try to give them 2 weeks at least to harden off, but I might be too cautious. I coddle the tomatoes & cucumbers a lot because we have a lot of cloudy 50 degree days yet in spring. It's tricky to have the starts ready, but not pot bound by the time the soil has warmed up enough.

As long as you are past freezing temps., a good way to grow plants is to put them outdoors as soon as they germinate. I do this when I start seeds. Once they germinate in my kitchen, I put them into small pots, in my unheated greenhouse. They don't skip a beat in their growth and that way there's no back and forth hardening off process to bother with. They are completely acclimated to the environment you're planning to plant them in by doing this. This can be done even if you don't have a greenhouse as I'm only using mine to house the plants for a short time. As long as the tiny seedlings aren't in a spot where a rabbit or other animal might eat them off, you can set them out on your patio, etc. to grow to the size at which you'll put them in pots or the ground.

Seedlings need to be hardened off gradually. You don't want to go from inside to direct sun or strong wind. I take mine out on the first day and put them into a few hours of bright light but not direct sun. The next day, I put them out into bright light but not direct sun for the entire day. The third day, I put them out for a few hours of direct sun and the fourth day I leave them in direct sun all day. I wouldn't expose them to radically different temperatures than what they are used to until they are hardened off. At best, that will stunt their growth and at worst it will kill them.

Kristin, It is hard to suggest to someone else what to do with their seedlings because none of us live in your exact climate/weather conditions nor do we know how strong your plants are, so we're just guessing. I usually harden my plants off first for a week on a screened-in porch where the screening provides some wind and sun protection. Then they go outside for 1 hr. the first day, 2 hrs. the second, 3 hrs. the third, etc. Once they are able to stay outside from sunrise to sunset, I leave them on the screened-in porch or in the potting shed which protects them from night-roaming plant-eaters like rabbits and deer but exposes them to cooler night air. If the temps are forecast to drop below 45 degrees, they go into the well-insulated barn. If a surprise snow storm or high winds arrive, they may find themselves spending a day or two in the garage. I never leave mine outside in the yard at night because if I did, the deer would throw a wild party and invite all their friends and I wouldn't have any seedlings by the next morning. I'd say that if you feel comfortable leaving them outside at night, go ahead, but be aware that any wild things, including domestic cats and dogs, raccoons, squirrels, rabbits, etc. roaming around at night may 'visit' them, and sometimes you have surprising number of animals roaming even in suburban areas. Also, always trust your instincts about the weather and do not trust the forecast. I have had a forecast of an overnight low of 50 lull me into a false sense of security only to have plants damaged when the overnight low dropped to 32 degrees. I learned an important lesson there. If your wind is forecast to blow higher than maybe 12-15 mph all night, I wouldn't leave them out. They need a break from the wind, and it has been relentless all week and should be strong again tomorrow. Dawn