There are two hotels in Wako-shi, both within walking distance of RIKEN and Wako-shi station.
Super hotel:
http://www.superhoteljapan.com/en/s-hotels/wako/
Toyoko Inn:
https://www.toyoko-inn.com/index.php/eng/search/detail/00090.html
Further afield, are hotels around Ikebukuro station. This train station has excellent links to Wako-shi.
As an example, check here: