One idea is a map pin board, one which you place a pin in every place you have gone to. This is a gift they can have for life and use to plan where they go next- especially if one continent is looking particularly sparse. You can get them for around £30 for the cheaper side of them, which you could split with some other friends.
Another idea is a scrapbook and some basic supplies, you could maybe even do the first page if you’ve been on holiday together. This can be cheap with the supplies only going to max £10 with the book- just go to any basic pound shop. This can be something you could do together if your friend is also your main travel buddy or something to help them keep the memories of their travels.
No matter what you gift your friend, anything is sentimental if it comes from a good place- even on a budget
The next few are more practical gifts, gifts with less excitement but things they could use on their travels. A travel wallet, a simple and sleek (so easy to store in a pocket or handheld luggage) wallet which holds all cards, ID, passport so you don’t hold up time in the airport. These run for about £13 for a basic one.
A clear toiletries bag. we all have that friend who forgets to put their toiletries in a clear bag and then end up with too many, help them out with a clear bag (in the right size), these are also very cheap (£5) and you can fill it with their favourite sweets when gifting it.
No matter what you gift your friend, anything is sentimental if it comes from a good place- even on a budget. The point is that a good gift can be practical or something more “abstract”, a good friend will just love the fact you got them something to do with their interest over how much you spent.