anything you like,you can sow veg seeds, and also some flowers,the seed packet will tell you about time to plant and also after care,,,,,,stroll

Try sweetpeas and Lupins as they are very easy and don't mind the cold. This type of greenhouse isn't frostproof but these seeds won't mind. Also anchor it down, may be something heavy on bottom shelf as mine blew to other side of garden in January, scattering my bits and pieces everywhere and had to start again.

Just a warning on small greenhouses. While they are very cold over night, they can heat up tremendously during the day. More seeds are lost by getting too hot on a sunny day than by not getting adequate temperatures.
Therefore, when sowing things in the early spring, make sure your greenhouse is well shaded on sunny days.
Just to give you an example, the temperature in our small greenhouse reached 45C a couple of days ago while the outside temperature was about 14C. I suspect many of the seeds I'd sown have 'cooked'.