Public transportation is much more popular and accessible in Europe so your parents shouldn't have any problem getting around. Also, European countries tend to be smaller so travel time is reduced.
There are a lot of travel opportunities for senior travelers looking to travel in groups. A good place to start would be at your local senior center. I found one great resource here www.eldertreks.com.
Orbitz is a good website to book online travel for seniors. They also have good discount rates.
Where you travel depends on your abilities and what you want to do. There are special travel plans for seniors with different activity levels and needs, and talking to a travel agent may help you find one that's right for you. There's also information available here on different options: www.suddenlysenior.com/TRAVELPAGE.html
Single seniors who like to travel now that they are retired and who want the company of other seniors for safety and social reasons can try looking at seniortravel.com for many ideas.
There are several websites that gives off good travel packages in general. This website, www.seniortravelhub.com/senior-travel-deals/travel-packages, is geared specifically towards seniors.
The main place for travel insurance for the elderly would be AARP which is well know for providing seniors with much help. Other insurance companies offer good deals for travel insurance.
Most insurance companies offer travel insurance. It's illegal to decline travel insurance base on age. A good place to look is a travel agency or AAA. Traveler's insurance is also a good one.
Travel insurance is a good idea for anyone. There are many places that provide travel insurance for seniors. One such place can be found here: http://travelinsuranceforseniors.org/travel-insurance-over-70
Insure for All offers a good amount of insurance for seniors 65 and older. Also, this company is created just for seniors who specialize in traveling. For more information you can see their site at www.insureforall.com.
Expedia offers wonderful travel advice. Some of their specials cater specifically to senior travelers. This is helpful as seniors do not like to search for these offers, having them in one place is good.
This website offers information as well as great deals for travel in Ontario http://www.senioryears.com/canadiandeals.html. It offers information on parks as well as rail travel so they can see a variety of attractions. Canada can be a wonderful place to travel.
It depends on your budget, but there are good deals out there for seniors. You can look at some of the deals at http://www.vacationidea.com/deals/deallv41207.html