How should we celebrate?

A playlist with his favourite music. We had great fun at my mother's 90th singing along to some of the music of her era.

A visit from a local musician or comedian or other people former...something specific to his tastes.

If your family like silly games....Divide the guests into two or 3 teams. Give each team a bag of marshmallows and some sphaghetti Each team has to make a tower using these 2 things which will be judged on architectural design and height. Give them about 8 or 9 mins. Is fun and team building and suitable for all age groups

A slideshow of his photographs sounds great, you can use even the old ones, find everything you can and digitize the pictures. Some of them may be faded, so you'll probably need to restore them a bit in Photoglory or Phowd, but this is a one click thing.
Here are some Father's day slideshow ideas- smartshow-software.com/fathers-day-slideshow-ideas.php This isn't a Father's day (obviously), but you may use some tips from the list, like the quotations or soundtrack ideas.
Also encourage all the guests to share stories about your father, i.e. how they met each other, what was the first impression, maybe something funny that had happened, etc.

The slideshow programme above looks fab!

You could do a playlist with songs for each decade of his life suggested by members of the different generations, working backwards.

How lucky you are to have a fit and healthy 95 year old father.

Quiz is always good. Make it a quiz about the birthday boy, though. I once did one for a friend with one section on the year of her birth (what was no. 1, who was football champion etc) and one section on the town she came from. That way it wasn't too personal and everyone could contribute.

You can have personalised pens, notebooks, beer mats, glasses or mugs made.

If there's time, collect up photos of him throughout his life and make a souvenir photo book. Leave some pages blank. Get the guests to sign the book and write in their good wishes. Take a photo of them whilst they are signing it and print them out afterwards and stick them next to the signatures. A lovely souvenir for everyone to admire.

Failing that, just get a small, bound book and let people write their birthday wishes and maybe an anecdote about the birthday boy. You could stick photos in later.

What fabulous suggestions you've all made! Thank you so much - I'm going to try and use most of them