barne,
I have operated a C150 in Switzerland for almost 13 years before stepping up to a M20 recently and flown all of it (and as far as Spain outside it) without any problems worth mentioning. It will take a reasonably long time getting here and back, doing some 90 kts and 300 NM legs at most, but it's perfectly feasible. I've done some crazy stuff like going Zürich to Spain and back in one day, but take a pain killer for your behind
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As to airports, coming from France, Grenchen has been mentioned which is a good port of entry. After that, there are many nice places to go to. In the Western region of Switzerland, Yverdon is nice, so is Lausanne. You can go to Sion, it involves mountains left and right but has a quite nicely big valley in front
. Bern is a good GA place, quite busy at times. Others are Wangen-Lachen on the lake of Zürich and St.Gallen/Altenrhein in the East. Zürich is feasible, mind that you need slots and good knowledge of ATC, otherwise Birrfeld is a possibility.
With a C150, mountains are not too big a problem as long as the plane is not too heavy and temps not too hot. Flying down to the Ticino via the Gotthard is fun if the weather is really nice (and only then for someone with no experience in mountains) and requires about 9000 ft over the Gotthard pass. Samedan is a nice place but not necessarily for a C150, even though I have done it during pilot training (in the early 80ties), I'd say you need an instructor with you for that one. Otherwise, stay north of the Alps but that means you can fly along them, pretty nice and scenic too.
I don't see many (if any) of these places allowing you to camp there, but there are inexpensive places around, just ask at the C-Office.
If you need specifics, PM me, I'll try to get you answers.
Best regards
AN2 Driver