Build a 2 month itinerary for me (March/April)

Hey all! I'm currently in Greece but looking to plan the rest of my trip through late April, maybe longer. Any suggestions if you had the freedom to plan two months of budget travel during March and April? I've listed below a few things to keep in mind:

- Need to remain budget conscious averaging <$35 per day. I've been able to do this pretty easily in Greece but have some concerns elsewhere in Europe. Looking to spend less than $2500 during this time.
- I have endless SkyMiles and CC points to cover expensive flights/trains, which really opens up a lot of possibilities including leaving Europe.
- Open to volunteer/work opportunities that could help burden the cost, extend the trip, and provide the benefits of enjoying the company of a group rather than riding solo at all times.
- If I spend the whole time in Europe, my 90 day Schengen Area limit will be late April.
- Outdoors recreation a huge plus. Would love to find some multi-day hikes. Balancing this with city life and sightseeing is important though.
- If I stay in Europe, I have friends in Vienna, Italy, London, and Spain whom I could visit and build an itinerary around.
- I've spent limited time in the UK, Paris, Barcelona, and Amsterdam, but that's about it in Europe.

Look forward to the responses. I definitely plan to post a trip report and will share my experience in Greece as well. Thanks!