That depends on whether or not you successfully solo'd on the Nexstar...

If you crashed the Nexstar and want a good airframe to finish training with:

Hangar 9 Solo Star/Solo Sport/Solo Strike ARF
Seagull Models Arising Star ARF
Tower Hobbies Tower Trainer .40 MkII ARF
World Models Sky Raider Mach I ARF

If you successfully solo'd and are looking for an easy to assemble second aircraft:

Thunder Tiger Tiger Stick .40 ARF
World Models Sky Raider Mach II ARF
Phoenix Models Dolphin .40 ARF

These are all great, inexpensive airframes that should go together quickly and exactly as per instructions.

If you're really building challenged and you want a bolt-together airframe as a replacement primary trainer:

Global Hobbies RCM Trainer .40 ARF
Thunder Tiger Tiger Trainer .40 Mk II ARF

These two airframes require no gluing and bolt together very quickly.

Good shopping and good luck.