One exercise I like is in hanmi-handachi, copying 'exactly' from the sequence that Ueshiba does (which is obviously a training sequence from aikijitsu).
from contact (shomen uchi, or just shomen-uchi type contact)
1. ikkyo irimi (each side) (uke weak)
2. ikkyo tenkan (each side) (uke strong & pushing through)
3. tenchi-nage - tenkan (uke strong & pushing through and resisting ikkyo tenkan. Basically you allow a little force towards you but you then move to the opposite side (that you would doing ikkyo tenkan) thus uke is thrown quite hard if he is resisting ikkyo tenkan).
*4. nikkyo (doing tenkan ikkyo 1st and using your thumb to lead around into nikkyo)
*5. sankyo (but this time grabbing ukes hand instead of wrist as you go into ikkyo tenkan type movement so sankyo is on with the other hand before you swap over).
Sorry only (*) is relevant to your question, but I thought I'd give you the sequence anyway.
P.S. if you grab ikkyo with a gokyo type grip, nikkyo is then possible - I think its useful to consider that all these techniques blend into one another, and not to think that nikkyo is one way or another; ideally you need to be able to do both ways and all ways in between!