The program BARI (Baltic Science Network Mobility Programme for Research Internships, https://www.baltic-science.org/bari/) offers a platform for doctoral students who want to temporarily integrate a foreign student into their doctoral project and students who want to gain practical experience abroad in a real research project.
PhD students in Bonn can propose projects in which students will assist in research and lab work. More details here: https://www.baltic-science.org/bari/
Participating countries are
Denmark, Germany, Estonia, Finland, Iceland, Latvia, Lithuania, Norway, Poland, Sweden and Russia (Belarus with observer status)