Care to share some general travel advice for me and my friend?

A couple of years ago I met someone online, we got talking regularly and are now quite good friends.

My friend recently moved, and is currently in the UK, working as a volunteer for people with Alzheimer.

In 4 months time she is going to come to Spain to visit me for a few days, then we will both go to Holland to visit one of my other online friends.

We would also like to perhaps stop off in France or Germany, or perhaps some other place.

What are some good places to visit?

How do we keep everyone safe?

How do we avoid travel problems?

What other advice do you have for us?