These go in ascending order of improbability.
1) Open to European trade.
The limitations on trade were popular and, in the context of 17th century Japan, made good economic and political sense.
2) Adopt European weapons
Up until 1750 or so, European weapons weren't much better than what they had anyway. Even after that, there's just no incentive to do so -- the Shogunate faced no external threats, and their cannon and muskets were more than adequate to control their own population.
3) "Adopt European technology"
That one is in quotes because it's usually a sign that the poster has no idea what he's talking about. Two reasons.
First, up until the middle 1700s, Japan wasn't behind Europe in any way that mattered. Their agriculture, small scale manufacturing, architecture, metallurgy and engineering were all just about as good. They used about as much coal and water power. They had clocks and eyeglasses. They knew about calculus, the moons of Jupiter and the circulation of the blood. Until the Industrial Revolution, there wasn't much that Europe had that Japan would have found useful.
Second, when something clearly useful did appear -- the telescope in the 1600s, the sweet potato in the 1700s -- they adopted it enthusiastically. "Rangoku", the study of European knowledge, was a recognized field of study from the early 1700s onward; it was slightly disreputable but tolerated exactly because it *did* sometimes throw up interesting and useful bits.
4) Expand beyond the home islands.
There was no compelling reason for them to do so, and several good reasons not to.
5) Become Christian.
Christianity offered no advantages over Buddhism, and came with some nontrivial negatives.
6) Conquer China.
Pre-Tokugawa Japan couldn't even conquer Korea.
There's a narrow window of a decade or two around the Ming-Manchu transition when Japan could have gone fishing in the troubled waters of the mainland. They could perhaps have grabbed (backwards, tribal) Taiwan, or even grabbed a coastal province or two. But, in the long run, this would have been a Very Bad Thing for Japan. The Tokugawa shoguns were bright enough to realize that, and so scrupulously stayed out.
-- In general, a lot of WIs about Tokugawa Japan suffer from the idea that the Shogunate and its policies were somehow pathological and "wrong". There's this idea Japan "should have" done otherwise -- opened to world trade, gone Christian, embarked on conquest, or whatever.
In fact, in retrospect it looks like the bakufu got almost everything right: they gave Japan 250 years of peace and prosperity while laying the foundations for subsequent rapid modernization. Even in 1850, Japan was perhaps the most advanced and prosperous Old World society outside of western and central Europe.
It's very easy to see how the Shoguns could have done worse, but it's actually rather hard to do much better. There are a number of WIs that "break" the Tokugawa Shogunate -- Ieyasu loses at Sekigahara, the Heian Uprising succeeds, etc. -- and most of these lead to a Japan that is poorer, more divided, more violent, and generally less ready to face the modern world.
Doug M.