Baby carrier / travel bed

My youngest child is now 14 Weeks old, he is going to be going into his big sisters Britax Dualfix car seat soon as she is going into a bigger size seat.  (the Dualfix looks far safer and sturdier than the seat from my travel system, which is a Venicci) Looking for some advice as this replaces a car seat from a travel system and am not sure what is best to put the child in when he is not in the car seat.  

As his new seat is fixed in the car if say we go visiting we will need to take him out of the seat and put him in something else?  A travel bed or carrier or something else more suitable?  Any recommendations would be very gratefully received?

Many thanks

Rosie