As far as I understand it, it is how long they are in there that is the issue. My HV said 15-20 mins a day is fine. All day is not fine. I think it is the same for anything that holds them in a certain position.
The answer to your question is most likely nothing that artificially supports them in a position that they are not developmentally ready for? Mine used to have a largish washable playmat/ picnic rug would do (joules do nice ones, which later just become useful for picnics/ beach trips etc) and things that are interesting to look at/ wriggle and crawl towards. As mine got to point of pulling up, they used a sofa and we were advised to encourage ds1 (who walked a bit late) to play with toys that were on the sofa seats whilst he stood next to it, to strengthen his muscles. He could sit or cruse at leisure. I think a. Ice present for a child aged 6-12m, although will likely last longer, is a treasure basket. You can put these together with ‘Home items’ and they are lovely open ended play. (Not sure this really answers your question?)