Just a mixer, some Android Devices and applications that have a sequencer built in to it and just JAM!!!
Think that soon everybody will have a couple of devices at least and even the old ones run at least Nanoloop which have a six track sequencer. So for example you could be running Heat Synthesizer on your high end device and Nanoloop on your old phone or just borrow a phone and download any of the free Synthesizer / Sequencer applications on the borrowed phone.
A four channel mixer can be found for 20-40 $/€ second hand...
Remember Heat Synthesizer is midi compliant if your device can handle it and you have a OTG USB cable. Can also use bluetooth typing keyboards!