Day trip from Tokyo - alternative to Nikko

Hello,
I am looking for an alternative to Nikko as a day trip from Tokyo. Hacking a toddler i am afraid it may be too far for just a day trip so i am looking for something about 1,5 h far by train. We are going to Kamakura but are hoping to find a second city to visit too.
We would love something historically relevan/full of temples but are willing to consider all options.

Thank you so much!

You can consider Enoshima or Izu or Hakone

There are dozens of possibilities - Hakone. the Fuji 5 Lakes, Enoshima, Kawagoe, Karuizawa, Chichibu, Nokogiriyama, Takao, Okutama, Yokohama, the Ushiku Great Buddha, and many others. But a few involve some hiking. You really won't find something filled up with so many temples in the region like Kamakura though.

Not to mention some places in Tokyo-to just outside the city, like the Showa Kinen Park, and Edo-Tokyo Open-Air Architectural Museum. They needn't take up a whole day though.

Just to add, Shibamata in Tokyo itself might also tick a few of these boxes.

Besides the Showa Kinen Park and Edo-Tokyo Open Air museum mentioned by Ken, the Nihon Minka-en (Japan Open-Air Folk House Museum) in Kawasaki (right next to Tokyo) is a great place to see old houses. They have done a great job of collecting, restoring, and curating the exhibits, with some work still in progress. For old buildings, I also really liked Kawagoe.

But if youfre talking about more distant history, and especially temples, in the Kanto area there really is no equivalent for Kamakura. And just as Kyoto is well worth more than a couple of days, Kamakura is well worth more than one. If youfre seriously into temples, it has more (that are well worth visiting) than you can fit into one day, especially with a toddler in tow. I wouldnft do two days in a row there (you could get gtempled outh) but maybe with a day or two between. (Try to avoid weekend days if you can, though.) Besides the most famous places near Kamakura Station, look into the sites near Kita Kamakura Station. I have had some excellent visits to temples in that area. Also, the Shonan monorail (Kamakura-Enoshima) looks great and your toddler might find it fun. (I havenft taken it myself yet but itfs on my to-do list.) And there is an aquarium in Enoshima. So there is no reason you canft go to Kamakura twice. It is easy to get there from Tokyo. (By the way, if you arenft on a tight budget, it can be worth upgrading to green car tickets if you are using one of the JR commuter trains that have them. Otherwise you might have to stand up. You can purchase the upgrade using a Suica card on the platform. Many people wouldnft spend the money, but compared to something like the price of a hotel in Shinjuku (!) it is peanuts. And a great deal more comfortable.)

I was asked for some easy day-trips by a guest as they have a couple of extra days in Tokyo this week. Showa Kinen Park was one of the recommendations as access from their accommodation in Shinjuku is super easy. I also suggested Yokohama (China Town, Red Brick Warehouses and Minatomirai area). These were easier and quicker than somewhere like Kamakura/Enoshima that can take a couple of hours to get to/from that they were considering.
There are (of course) a lot of options if prepared to travel a couple of hours for Nikko alternatives, but longer distances cost more, even if similar travel times to some local places.

Are you really looking for religious temples? Or are you looking for old wooden structures?

The forementioned EDO-TOKYO OPEN AIR ARCHITECTURAL MUSEUM
https://www.tatemonoen.jp/english/

as well as the JAPAN OPEN-AIR FOLK HOUSE MUSEUM
https://www.nihonminkaen.jp/

are both vast parks that's more like a village where your toddler would be free to run around for a day, and would have less steps (stairs) compared to temples which tend to be built on hills. Both museums are located at a different direction from Kamakura, so it would be worth spending a different day.

Meanwhile, if you're really looking for places "full of temples", that would be a place where it was once an ancient city (because in rural places, you'd only need one temple). So, you need to travel further to a certain extent if you decide to escape Tokyo. But neighborhoods like Yanaka (surrounding JR Nippori Station), within the city of Tokyo, is full of temples including the one with the grave of Tokugawa Shogun (which is in the temple of Kaneiji). The cute little Megurin Bus would help you travel through local streets in the area.
https://www.city.taito.lg.jp/kenchiku/kotsu/megurin/index.html

I hope it helps.