Hello,
I wish that in the table of contents and only one, the number of chapters
are I, II, III, IV ...
I would also have this:

I First chapter
   Activity
   1.1 Second section
   1.2 Third section

thanks,
Fabrice

\setuplist
  [chapter]
  [after={\blank[line]},
   style=bold]

\setuplist
  [section]
  [after={\blank[line]}]

\setupcombinedlist
  [content]
  [aligntitle=yes,
   alternative=c]

\starttext

 \title [content] {Sommaire}

\placecontent

\startchapter[title={First chapter}]

\startsection[title={Activity},number=no]

\stopsection

\startsection[title={Second section}]

\stopsection

\startsection[title={Third section}]

\stopsection

\stopchapter

\stoptext