Don't assume that this place is just an onsen--there are games, massages/spa treatments, a wide variety eateries, etc. I would schedule at least 2 hours, but if you're prepared to take it all in, 4-5 hours is a good amount of time there.
It's a beautiful shrine on its own, but be sure to plan a same-day trip out to the Kamiiso-no-torii Gate along the coast line to maximize your time in the area!
Mito offers a lot of historical attractions, so this is a nice complement to that. Be sure to plan to visit the Art Tower if you're in the city, and check the website for event schedules. Occasionally, they will have recitals and concerts that are free for public viewing.
If you are staying in Tsukuba and want a nature retreat, head out to Kasumigaura-ko. It provides a wonderful contrast to the modern city of science and is only a relatively short trip away.
This is not unlike many other fish markets you'll visit, but as always, it is best to look for the local specialties. For Ibaraki, try to find the Anglerfish!
This is a fun place to get some hands-on experience, but be sure to reserve a spot ahead of time (check the website for details). Also, you'll need to provide an address for them to ship the completed item after it's fired (usually takes a couple of weeks to get to you).
One of the most photogenic spots in Ibaraki. I recommend checking the tide schedules to maximize your viewing (and photographing) experience.
This is a quirky place. If you are not a fan of Mentaiko, it probably won't offer too much for you, but if you even remotely enjoy it, this can be an extremely interesting and fun place to visit.
Tsukuba is the science hub for all of Japan (a deliberate move by the Japanese government), so the Expo Center is an awesome place to see what sort of innovation and discovery has taken place there.
This place is beautiful. More than just a statue, there are well-manicured gardens, a museum on site for other interesting artifacts, and a modest shopping area with local treats and souvenirs. Also, if you take a picture from a distance, it looks like the Buddha is doing a Kamehameha from Dragonball.