I am taking a few routes that do not require seat reservations. Do I need to reserve a place on the train beforehand?

Without the mentioning these exact routes, date and times, it is impossible to say.

But the answer is probably “no”. Or you just find a free seat in trains without any form of reservation, or you pay a small amount for seat reservation on trains with optional seat reservations (in countries like Germany, Austria, Czechia,...).

If you are travelling in the UK any train with the opportunity to reserve a seat is free (except Eurostar and sleepers) and many can be sourced from the GWR website.

https://www.gwr.com/your-tickets/seat-reservations

You will need to set up an account.

For LNER (London to the North and Scotland) reservations are advised and can be sourced online at https://www.lner.co.uk/travel-information/make-a-reservation/

I will be going from Amsterdam to Geneva. When I go to the seat reservation area on the Eurail website, it shows me that the route is included. So can I just activate my travel day and hop on to the train?

Yes if you go through Germany, there's no need to reserve but trains can be busy, on a Friday for example or generally in summer.

If you go via Paris, there are (very expensive) seat reservation fees.

You can always check a couple of days, maybe a week, before on bahn.com. Expected demand is shown. If demand would be high, or very high a seat reservation is recommended. You can get these on the same website (choose seat reservation only).

But if you travel alone, I wouldn’t bother. Always a free seat to be found, when travelling from the 1st station. If you travel in a small group, it’s quite advisable to reserve some seats though.

With Deutsche Bahn, if a seat is not taken after 15 minutes, the possible reservation is cancelled. After that, anyone can sit there.

In Switzerland no reservations are needed, except on some tourist scenic trains.

Hi @Hrithik Raj Agarwal 

Please keep in mind that we are still waiting for a definite answer from NS and ICE regarding trains from the Netherlands towards Germany for the months of June, July and August. 

Have a nice day! 

Consumers’ organisations are still waiting for the promised consultation…

But NS has already said that you will be fine if you've booked a reservation in advance.

At this point there is no official outcome. Where did you find this information? 

Correct, there is no official outcome yet. I'm just telling what I know.