It is advised that whilst receiving treatment you do not expose yourself to sun rays as chemotherapy can make your skin hyper sensitive. It would be best to not make any holiday plans as it cannot be specified how long your treatment will last. The reasons being that if you visit abroad and you encounter bleeding or other side effects from your condition/treatment you will not be able to gain the specialised treatment that your regional centre can give. It has also been known that you could be more at risk of developing DVT on long haul flights whilst receiving chemotherapy.
You may also find it difficult to gain travel insurance because trophoblastic disease is classified as a critical illness/pre malignancy as it is linked to cancer and some insurers may not cover this. (Please consult individual insurance companies to see if this is the case)
BUT, if you are coming to the end of your treatment, you feel well enough and have discussed it with your consultant there is no reason why you cannot take short breaks in between treatments.
It is also advisable to declare your illness with your individual vehicle insurer, as you may not be covered or they may class you as being at a higher risk whilst you are receiving treatment.